Internal Combustion Engine

Internal Combustion EngineAn Introduction:

An internal combustion engine is any engine that operates by burning its fuel inside the engine. In contrast a steam engine burns its fuel outside the engine. The most common internal combustion engine type is gasoline powered. Others include those fueled by diesel, hydrogen, methane, propane, etc. Engines typically can only run on one type of fuel and require adaptations to adjust the air/fuel ratio or mix to use other fuels.

In a gasoline engine, a mixture of gasoline and air is sprayed into a cylinder. This is compressed by a piston and at optimal point in the compression stroke; a spark plug creates an electrical spark that ignites the fuel. The combustion of the fuel results in the generation of heat, and the hot gases that are in the cylinder are then at a higher pressure than the fuel-air mixture and so drive the piston back down. These combustion gases are vented and the fuel-air mixture reintroduced to run a second stroke. The outward linear motion of the piston is ordinarily harnessed by a crankshaft to produce circular motion. Valves control the intake of air-fuel mixture and allow exhaust gasses to exit at the appropriate times.

Types:

1-Four-Stroke IC Engine2-Two-Stroke IC Engine

1-Four-Stroke IC Engine:Engine Diagram:

Engine Operations:

It is a good engine to learn the fundamentals of engine operation. This type of internal combustion engine is called a four-stroke engine because there are four movements, or strokes, of the piston before the entire engine firing sequence is repeated. The four strokes are described below with some still figures. In the animation and in all the figures, we have colored the fuel/air intake system

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red, the electrical system green, and the exhaust system blue. We also represent the fuel/air mixture and the exhaust gases by small colored balls to show how these gases move through the engine. Since we will be referring to the movement of various engine parts, here is a figure showing the names of the parts:

Intake Stroke

The engine cycle begins with the intake stroke as the piston is pulled towards the crankshaft (to the left in the figure). The intake valve is open, and fuel and air are drawn past the valve and into the combustion chamber and cylinder from the intake manifold located on top of the combustion chamber. The exhaust valve is closed and the electrical contact switch is open. The fuel/air mixture is at a relatively low pressure (near atmospheric) and is colored blue in this figure. At the end of the intake stroke, the piston is located at the far left and begins to move back towards the right. The cylinder and combustion chamber are full of the low pressure fuel/air mixture and, as the piston begins to move to the right, the intake valve closes.

Compression Stroke

With both valves closed, the combination of the cylinder and combustion chamber form a completely closed vessel containing the fuel/air mixture. As the piston is pushed to the right, the volume is reduced and the fuel/air mixture is compressed during the compression stroke. During the compression, no heat is transferred to the fuel/air mixture. As the volume is decreased because of the piston's motion, the pressure in the gas is increased, as described by the laws of thermodynamics. In the figure, the mixture has been colored yellow to denote a moderate increase in pressure. To produce the increased pressure, we have to do work on the mixture, just as you have to do work to inflate a bicycle tire using a pump. During the compression stroke, the electrical contact is kept opened. When the volume is the smallest, and the pressure the highest as shown in the figure, the contact is closed, and a current of electricity flows through the plug.

Power Stroke

At the beginning of the power stroke, the electrical contact is opened. The sudden opening of the contact produces a spark in the combustion chamber which ignites the fuel/air mixture. Rapid combustion of the fuel releases heat, and produces exhaust gases in the combustion chamber. Because the intake and exhaust valves are closed, the combustion of the fuel takes place in a totally enclosed (and nearly constant volume) vessel. The combustion increases the temperature of the exhaust gases, any residual air in the combustion chamber, and the combustion chamber itself. From the ideal gas law, the increased temperature of the gases also produces an increased pressure in the combustion chamber. We have colored the gases red in the figure to denote the high pressure. The high pressure of the gases acting on the face of the piston causes the piston to move to the left which initiates the power stroke.

Exhaust Stroke

At the end of the power stroke, the piston is located at the far left. Heat that is left over from the power stroke is now transferred to the water in the water jacket until the pressure approaches atmospheric pressure. The exhaust valve is then opened by the cam pushing on the rocker arm to begin the exhaust stroke. The purpose of the exhaust stroke is to clear the cylinder of the spent exhaust in preparation for another ignition cycle. As the exhaust stroke begins, the cylinder and combustion chamber are full of exhaust products at low pressure (colored blue on the figure above.) Because the exhaust valve is open, the exhaust gas is pushed past the valve and exits the engine. The intake valve is closed and the electrical contact is open during this movement of the piston. At the end of the exhaust stroke, the exhaust valve is closed and the engine begins another intake stroke.

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Classification:

1-Physical Structure Base 2-Overhead Cam Engine 3-Cam Shaft directly Operated Valve

4-Carborator Base 5-EFI

6-Fuel-Injection

i. Single point injection ii. Multi point fuel injection iii. Gasoline direct injection

iv. HPGD Injection

7-Fuel Injector

8-Ignition System

i. Magneto Ignition systemii. Battery

iii. Transistoriv. Distributor less

v. Laser

9-Air Intake

i. Naturally Aspirated ii. Turbo/Supercharged

Supercharging:

One way to increase engine power is to force more air into the cylinder so that more power can be produced from each power stroke. This can be done using some type of air compression device known as a supercharger, which can be powered by the engine crankshaft.

Supercharging increases the power output limits of an internal combustion engine relative to its displacement. Most commonly, the supercharger is always running, but there have been designs that allow it to be cut out or run at varying speeds (relative to engine speed). Mechanically driven supercharging has the disadvantage that some of the output power is used to drive the supercharger, while power is wasted in the high pressure exhaust, as the air has been compressed twice and then gains more potential volume in the combustion but it is only expanded in one stage.

Turbo charging:

A turbocharger is a supercharger that is driven by the engine's exhaust gases, by means of a turbine. It consists of a two piece, high-speed turbine assembly with one side that compresses the intake air, and the other side that is powered by the exhaust gas outflow.

When idling, and at low-to-moderate speeds, the turbine produces little power from the small exhaust volume, the turbocharger has little effect and the engine operates nearly in a naturally-aspirated manner.

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When much more power output is required, the engine speed and throttle opening are increased until the exhaust gases are sufficient to 'spin up' the turbocharger's turbine to start compressing much more air than normal into the intake manifold.

Turbo charging allows for more efficient engine operation because it is driven by exhaust pressure that would otherwise be (mostly) wasted, but there is a design limitation known as turbo lag. The increased engine power is not immediately available; due to the need to sharply increase engine RPM, to build up pressure and to spin up the turbo, before the turbo starts to do any useful air compression. The increased intake volume causes increased exhaust and spins the turbo faster, and so forth until steady high power operation is reached. Another difficulty is that the higher exhaust pressure causes the exhaust gas to transfer more of its heat to the mechanical parts of the engine.

Compression Ratio:

Compression ratio, in an internal-combustion engine, degree to which the fuel mixture is compressed before ignition. It is defined as the maximum volume of the combustion chamber (with the piston farthest out, or bottom dead centre) divided by the volume with the piston in the full-compression position (with the piston nearest the head of the cylinder, or top dead centre). A compression ratio of six means that the mixture is compressed to one-sixth its original volume by the action of the piston in the cylinder. The maximum possible ratio based on cylinder dimensions may not be achieved if the intake valve closes after the piston begins its compression stroke, as this would cause backflow of the combustible mixture from the cylinder. A high ratio promotes efficiency but may cause engine knock.

Higher compression ratio, Here, we are limited by auto ignition of the gasoline – knock. That is, if the gasoline engine compression is above about 10.5, unless the octane number of the fuel is high, knocking combustion occurs. This is annoying and if persistent, damage to the engine can occur. Thus, gasoline engines are limited in their efficiency by the inability of the fuel to smoothly burn in high compression ratio engines.

However, the diesel engine is not subject to this limitation. It runs at high compression ratio. In part, this explains its high efficiency. It also runs lean, and its pumping work is low, further increasing its efficiency over the gasoline engine. Humankind needs quiet, smoke-free, odor-free diesels

Vs=Swept volume

Vc=Clearance Volume

PV Diagrams:

A pressure volume diagram (or P-V diagram, or volume-pressure loop) is used to describe a thermal cycle involving the following two variables:

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Volume (on the X axis) Pressure (on the Y axis)

This is in fact enough information to fully describe a simple system from a thermodynamic standpoint. The diagrams are useful when one wants to calculate the work done by the system, the integral of the pressure with respect to volume. One can often quickly calculate this using the PV diagram as it is simply the area enclosed by the cycle.

Otto Cycle Diesel Cycle

TS Diagrams:

Otto Cycle Diesel Cycle

Methods Increasing of Engine Power:

1-Increase the compression ratio (Increase the power loop and decrease the pumping loop)

2-By increasing size of engine (Increase the swept volume)

3-By using better fuel (high calorific value of fuel)

4-By using Turbo-charger

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5-Rate of combustion should be increased

6-Increase the lubrication

7-Increase the intake valve of intake manifold

Variable valve timing and Lifting

VTEC (Variable Valve Timing and Lift Electronic Control) is a valve train system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine. This system uses two camshaft profiles and electronically selects between the profiles. Different types of variable valve timing and lift control systems have also been produced by other manufacturers (MIVEC from Mitsubishi, AVCS from Subaru, VVTL-i from Toyota, VarioCam Plus from Porsche, VVC from Rover Group, VVEL from Nissan, etc.).

In internal combustion engines, variable valve timing, often abbreviated to VVT, is a generic term for an automobile piston engine technology. VVT allows the lift, duration or timing (in various combinations) of the intake and/or exhaust valves to be changed while the engine is in operation. Two-stroke engines use a power valve system to get similar results to VVT. Piston engines normally use poppet valves for intake and exhaust. These are driven (directly or indirectly) by cams on a camshaft. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains.

The profile, or position and shape of the cam lobes on the shaft, is optimized for a certain engine revolutions per minute (RPM), and this tradeoff normally limits low-end torque, or high-end power. VVT allows the cam timing to change, which results in greater efficiency and power, over a wider range of engine RPMs.

An engine requires large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions. For this reason, pure racing engines which are designed to idle at speeds close to 2,000 rpm, cannot idle well at the lower speeds (around 800 rpm) expected of a road car.

Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to use VVT as a solution. Most simple VVT systems advance or retard the timing of the intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of cam lobes at a certain engine RPM. Furthermore Honda's i-VTEC can alter intake valve timing continuously.

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Carburetor:

The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.

When carburetors are used in aircraft with piston engines, special designs and features are needed to prevent fuel starvation during inverted flight. Later engines used an early form of fuel injection known as a pressure carburetor.

Most production carbureted (as opposed to fuel-injected) engines have a single carburetor and a matching intake manifold that divides and transports the air fuel mixture to the intake valves, though some engines (like motorcycle engines) use multiple carburetors on split heads. Multiple carburetor engines were also common enhancements for modifying engines in America from the 1950s to mid-1960s, as well as during the following decade of high-performance American muscle cars fueling different chambers of the engine's intake manifold. Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never "flooding" the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil-covered mesh; this was an effective system in a time when paper air filters did not exist. The main disadvantage of basing a Carburetor's operation on Bernoulli's principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed. The fuel jets are much smaller and limited mainly by viscosity, so that the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle. Most commonly this has been corrected by using multiple jets. In SU and other movable jet carburetors, it was corrected by varying the jet size. For cold starting, a different principle was used, in multi-jet carburetors. A flow resisting valve called a choke, similar to the throttle valve, was placed upstream of the main jet to reduce the intake pressure and suck additional fuel out of the jets.

Valve Overlap:

When we got it right we would have an exhaust pipe that would carry a positive pressure wave of exhaust pulse down the pipe to the open end. There it would collapse and create a negative pressure wave that would return back up the pipe. If the negative wave arrives back at the exhaust valve just before it closes, it will suck more of the exhaust gases out of the cylinder. This lowers the pressure inside the cylinder and makes the next intake stroke more efficient.

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On a 4-stroke, the intake valve begins to open while the exhaust valve is still off its seat. This is valve overlap. This allows the negative exhaust pulse (the reflection of the positive pulse) to actually pull more fresh mixture past the intake valve and into the cylinder. Here's how it works, and it has nothing to do with exhaust tuning as such.

When the combustion cycle begins, the piston is forced downward; this is the power stroke. Near the bottom of the power stroke the energy is mostly spent and the exhaust valve starts to open. It will actually start to open slightly before bottom dead center. The exhaust charge then begins to rush out the exhaust pipe.

The exhaust gases rushing out are further assisted by the piston pushing up on the exhaust stroke. This forms a stream of hot gas in very rapid motion away from the cylinder. This stream of hot gas has inertia and it will tend to continue moving in the same direction out the exhaust pipe even after the piston stops pushing it. This creates a region of reduced pressure in the vicinity of the exhaust valve.

By opening the intake valve just prior to top dead center, while the exhaust valve is still open (overlap), the gases going out the exhaust pipe will begin pulling the new intake mixture in behind them. Or, the intake stream will try to flow into the region of reduced pressure behind the exhaust stream, if you want to look at it that way. So overlap merely takes advantage of the inertia of the exhaust gases and the low-pressure region that it produces near the exhaust valve at the end of the exhaust stroke.

That part of the overlap design is common to all 4-stroke engines in order to gain additional charging of the cylinder with fuel mix at high RPM. The higher the RPM we design for, the greater the intake and exhaust overlap we build into the cam lobes. Most engines are fitted with exhaust manifolds that collect all the gases from a bank of cylinders. They also usually have a long pipe and muffler. So, while the physics of gases in motion will apply there, tuning for the exhaust pulse will not.

Engine Work:

Well Work is the integral of P*dV and since the crank angle plot doesn't directly show volume you need to calculate instaneous volume of the cylinder as a function of crank angle. If you know the bore and stroke you can find the volume of the cylinder and you can relate this to the crank angle. Once you have these values you can replace the crank angle with volume and have a P-V plot or pressure on the y-axis and volume on the x-axis. Then you can integrate the area under the curve and find the work from the engine. However, all this is done for you if you can find the P-V diagram for your engine or engine cycle (Sterling, Atkinson, Otto, Diesel, etc.) and integrate this for the work.

Crank Angle Diagram:

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2-Two Stroke IC Engine:In combustion engines the inner energy set free by combustion (e.g. of gasoline or diesel fuel) is changed partly into mechanical energy. Beside the four-stroke engine, the two-stroke engine is used (e.g. in lawn-mowers or in power saws). Every second stroke the engine is operating - there are no valves. It is necessary to use a mix of gasoline and oil (two-stroke oil) as fuel. This is used to lubricate the piston and the crank shaft.

The principle of the way the 2-stroke engine works:

1ST Stroke:

The compressed fuel-air mixture ignites and thereby the piston is pressed down. At the same time the intake port I is covered by the piston. Now the new mixture in the crankcase becomes precompressed. Shortly before the piston approaches the lower dead centre, the exhaust port and the overflow conduit are uncovered. Being pressurized in the crankcase the mixture rushes into the cylinder displacing the consumed mixture (exhaust now).

2nd Stroke:

The piston is moving up. The overflow conduit and the exhaust port are covered; the mixture in the cylinder is compressed. At the same time new fuel-air mixture is sucked into the crankcase. By means of a crank shaft the up and down motion is converted into a rotational motion.

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Description:

Stroke cycle is very widely employed where small power required for motor cycle, auto rickshaw, scooters. These types of engines are compact in size, easy for manufacturing and simple in operations. One drawback is there, Specific Fuel Consumption (S.F.C) is more.(means fuel per Break Horse Power (b.h.p.) per hour is more).

There are no inlet and exhaust valves as in four stroke engine but we have inlet and exhaust ports only, due to which suction and exhaust stroke are eliminated in two stroke cycle engine. Here the burnt exhaust gases are forced out through the exhaust port by a fresh charge of fuel which enters the cylinder nearly at the end of working stroke through inlet port. This process is called as "Scavenging". Details about Scavenging will be covered in another post.

As I told above, it has no valves but consists of the inlet port (IP), exhaust port (EP) and transfer port (TP).The ignition starts due to the spark given by spark plug when the piston be nearing the completion of its compression stroke. As a result, piston is pushed down performing the working stroke and in doing so; the air-fuel mixture already drawn from the inlet port in the previous stroke is compressed to a pressure of about 1.4 kilogram/centimeter square.

When 80% of this stroke is completed the exhaust port is uncovered slightly and some of the charge of burnt gases escapes to the atmosphere. As the exhaust port is uncovered by the further downward movement of the piston, the transfer port, which is slightly lower than exhaust port, is also uncovered and a charge of compressed air-fuel mixture enters the cylinder and further pushes out the burnt gases out of the exhaust port.

PV and TS Diagram:

2-Stroke Compression Ratio:

We calculate CR by taking the total volume (displacement plus head) and dividing that by the head volume alone. For a 70cc kit with a 7 cc head (stock 50cc) + gaskets ( 1-2 cc) it works out like (70+8)/8= 9.75:1.

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PSI can be an indicator of compression ratio, but requires much tricker math to actually figure out CR. Other factors like ring sealing, port dimensions, pipes, intakes, etc, affect the PSI reading.

The 'squish band' affects the 'v' or velocity part of the equation. Air moving quickly has more energy and explodes faster. Imagine having a bowl of vinegar and dumping in baking soda. If you stir the heck out of it, it will fizz up much more violently. That is what is happening as the piston approaches the squish band. The quick decrease in volume will blast all the gasses into the hemispherical chamber around the spark plug at the same time, the violence of moving air speeds the reaction. Jennings and Bell recommend the 40% of area- 7 deg taper method of sizing a squish band.

Because of the 'faster' flame in a high compression setup, ignition timing will have to be retarded. The ignition triggers and begins igniting the fumes before top dead center, to account for the time it takes to burn the air. When the everything burns faster, the max intensity of the explosion is at the wrong time and pushes the piston down as it comes up (knocking or pinging). This is exacerbated by a pipe, (or supercharger) which increases the pressure in the cylinder.

Sometimes the compression is just too high, the fuel ignites without any spark, and this is also problematic. Going to a higher octane fuel (only necessary in this condition) will allow you to continue to run the higher CR and pipe, otherwise you have to drop CR or go to a different pipe

The Effect of Higher Compression Ratio in Two-Stroke Engines:

The effect of higher compression ratio on fuel consumption and power output was investigated for an air-cooled two-stroke motorcycle engine. The results show that actual fuel consumption can improve by 1-3% for each unit increase of compression ratio over the compression ratio range of 6.6 to 13.6. The rate of improvement is smaller however as compared to theoretical values. The discrepancies are mainly due to increased mechanical and cooling losses, short-circuiting at low loads, and increased time losses at heavy loads. Power output also improves, but the maximum compression ratio is limited due to knock and the increase in thermal load.

Volumetric efficiency:

Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio (or percentage) of what quantity of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%. Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency

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will generally be able to run at higher speeds (commonly measured in RPM) and produce more overall power due to less parasitic power loss moving air in and out of the engine.

There are several standard ways to improve volumetric efficiency. A common approach for manufacturers is to use larger valves or multiple valves. Larger valves increase flow but weigh more. Multi-valve engines combine two or more smaller valves with areas greater than a single, large valve while having less weight. Carefully streamlining the ports increases flow capability. This is referred to as Porting and is done with the aid of an air flow bench for testing.

Many high performance cars use carefully arranged air intakes and tuned exhaust systems to push air into and out of the cylinders, making use of the resonance of the system. Two-stroke engines take this concept even further with expansion chambers that return the escaping air-fuel mixture back to the cylinder. A more modern technique, variable valve timing, and attempts to address changes in volumetric efficiency with changes in speed of the engine: at higher speeds the engine needs the valves open for a greater percentage of the cycle time to move the charge in and out of the engine.

Volumetric efficiencies above 100% can be reached by using forced induction such as supercharging or turbocharging. With proper tuning, volumetric efficiencies above 100% can also be reached by naturally-aspirated engines. The limit for naturally-aspirated engines is about 137%[1]; these engines are typically of a DOHC layout with four valves per cylinder.

Torsion bar Suspension:

A torsion bar suspension, also known as a torsion spring suspension and incorrectly as a torsion beam, is a general term for any vehicle suspension that uses a torsion bar as its main weight bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, mounted perpendicular to the bar, that is attached to a suspension arm, spindle or the axle. Vertical motion of the wheel causes the bar to twist around its axis and is resisted by the bar's torsion resistance. The effective spring rate of the bar is determined by its length, cross section, shape and material.

Torsion bar suspensions are currently used on armored fighting vehicles or tanks like the T-72 (Many tanks later in World War II used this suspension), trucks and SUVs from Ford, Dodge, GM, Mitsubishi, Mazda, Nissan and Toyota. Manufacturers change the torsion bar or key to adjust the ride height, usually to compensate for heavier or lighter engine packages. While the ride height may be adjusted by turning the adjuster bolts on the stock torsion key, rotating the stock keys too far can bend the adjusting bolt and (more importantly) place the shock piston outside the standard travel. Over-rotating the torsion bars can also cause the suspension to hit the bump stop prematurely, causing a harsh ride. Aftermarket forged torsion key kits use relocked adjuster keys to prevent over-rotation, as well as shock brackets that keep the piston travel in the stock position.

Advantages and disadvantages:

The main advantages of torsion bar suspension are durability, easy adjustability of ride height, and small profile along the width of the vehicle. It takes up less of the vehicle's interior volume compared to coil springs. A disadvantage is that torsion bars, unlike coil springs, usually cannot provide a progressive spring rate. In most torsion bar systems, ride height (and therefore many handling features) may be changed by simply adjusting bolts that connect the torsion bars to the steering knuckles. In most cars with this type of suspension, swapping torsion bars for a different spring rate is usually an easy task.

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Adjusting Torsion Bar Suspension:

Because of the torsion that torsion rods are subject to, they may require periodic adjustment in order to keep functioning properly. One of the most common problems that can occur with torsion bar suspensions is that the torsion rods in the suspension will begin to warp or sag slightly. This leads to the suspension not offering as much support for the vehicle as it once did and can result in an uneven ride and reduced gas mileage. Periodic adjustment of the torsion bars can eliminate this sagging and can also allow your mechanic to recognize excess wear in the torsion rods of your suspension so that you will know when the rods need to be replaced.

A Common List of Advantages and Disadvantages:Advantages of 2 Stroke Engines:Two-stroke engines do not have valves, simplifying their construction. Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost. Two-stroke engines are lighter, and cost less to manufacture. Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

Disadvantages of 2 Stroke Engines:Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon. Two-stroke engines produce more pollution. From: The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-

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stroke engine can emit more oily smoke.Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

Speed Governor:

A governor, or speed limiter, is a device used to measure and regulate the speed of a machine, such as an engine. A classic example is the centrifugal governor, also known as the Watt or fly-ball governor, which uses weights mounted on spring-loaded arms to determine how fast a shaft is spinning, and then uses proportional control to regulate the shaft speed.

Operation of a Governor:

A vital component of any diesel engine system is the governor, which limits the speed of the engine by controlling the rate of fuel delivery.

*Diesel engine speed is controlled solely by the amount of fuel injected into the engine by the injectors. Because a diesel engine is not self-speed-limiting, it requires not only a means of changing engine speed (throttle control) but also a means of maintaining the desired speed. The governor provides the engine with the feedback mechanism to change speed as needed and to maintain a speed once reached. A governor is essentially a speed-sensitive device, designed to maintain a constant engine speed regardless of load variation. Since all governors used on diesel engines control engine speed through the regulation of the quantity of fuel delivered to the cylinders, these governors may be classified as speed-regulating governors.

As with the engines themselves there are many types and variations of governors. In this module, only the common mechanical-hydraulic type governor will be reviewed. The major function of the governor is determined by the application of the engine. In an engine that is required to come up and run at only a single speed regardless of load, the governor is called a constant-speed type governor. If the engine is manually controlled, or controlled by an outside device with engine speed being controlled over a range, the governor is called a variable- speed type governor. If the engine governor is designed to keep the engine speed above a minimum and below a maximum, then the governor is a speed-limiting type. The last category of governor is the load limiting type. This type of governor limits fuel to ensure that the engine is not loaded above a specified limit. Note that many governors act to perform several of these functions simultaneously and cause the governor to shut down the engine. This provides the engine with a built-in shutdown device to protect the engine in the event of loss of lubricating oil pressure.

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Engine knocking:

Knocking (also called knock, detonation, spark knock, pinging or pinking) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive. It should not be confused with pre-ignition.

Knock detection:

Due to the large variation in fuel quality, a large number of engines now contain mechanisms to detect knocking and adjust timing or boost pressure accordingly in order to offer improved performance on high octane fuels while reducing the risk of engine damage caused by knock while running on low octane fuels. An early example of this is in turbo charged Saab H engines, where a system called Automatic Performance Control was used to reduce boost pressure if it caused the engine to knock

Knock prediction:

Since the avoidance of knocking combustion is so important to development engineers, a variety of simulation technologies have been developed which can identify engine design or operating conditions in which knock might be expected to occur. This then enables engineers to design ways to mitigate knocking combustion whilst maintaining a high thermal efficiency.

Since the onset of knock is sensitive to the in-cylinder pressure, temperature and auto ignition chemistry associated with the local mixture compositions within the combustion chamber, simulations which account for all of these aspects [7]

have thus proven most effective in determining knock operating limits and enabling engineers to determine the most appropriate operating strategy.

Common Reasons for Engine Knocking:

Improper Combustion Process:

An engine can ping (or knock) due to an improper combustion process. A "spark knock" is the result of combustion occurring too early. Early combustion can occur from carbon buildup inside the combustion chamber, a lean air/fuel mixture, and advanced ignition timing (spark plug firing too soon). In a properly-firing cylinder, the spark plug ignites the

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air/fuel mixture and a flame front starts on one side of the piston and burns across the top to the other side, which creates a rapid and evenly-expanding gas that pushes down on the top of the piston. When the air/fuel mixture is ignited prior to the spark plug firing, the two flame fronts collide, causing the pinging/knocking noise.

Engine is too hot:

An engine can ping because it is too hot. This is another uneven combustion scenario that is caused by the air-to-fuel mixture "lighting off" by itself. If the cooling system does not keep the engine's combustion chamber temperature in check, the air-to-fuel mixture will begin to spontaneously explode. This is also called "pre-ignition."

Improper Gasoline Octane:

In addition to cooling system problems, pinging can be caused by improper gasoline octane, an overly lean air-to-fuel mixture, or a lack of proper exhaust gas recirculation. The exhaust gas recirculation system (EGR) was created to neutralize engine pinging by adding a small amount of exhaust gas to the air-to-fuel mixture going in to the combustion process, which limits the peak combustion chamber temperature.

Internal Mechanical Problems:

Internal mechanical problems can also cause engine knocking. One such problem stems from excessive clearance inside the bearings in the connecting rods that transfer the downward movement of the pistons to crankshaft rotation. Each time the piston changes direction, there is a knock from the metal hitting metal. This is often referred to as a "rod knock." It is usually very rhythmic—it increases with engine speed and intensifies with engine load.

Other mechanical problems that lead to engine knocking are:

Defective main crankshaft bearings

A cracked or broken flywheel or flex-plate that attaches the engine to the transmission

A worn water pump bearing

A failed or loose timing belt tension can knock when the timing belt slaps against it

An air conditioning compressor can knock when it is failing or icing up

An alternator with worn rotor bearings can knock when the pistons fire

Fuel Octane Number and Rating:

Internal combustion engine power primarily originates from the expansion of gases in the power stroke. Compressing the fuel and air into a very small space increases the efficiency of the power stroke, but increasing the cylinder compression ratio also increases the heating of the fuel as the mixture is compressed (following Charles's law).

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A highly flammable fuel with a low self-ignition temperature can combust before the piston reaches top-dead-center (TDC), potentially forcing the piston backwards against rotation. Alternately, a fuel which self-ignites at TDC but before the piston has started downwards can damage the piston and cylinder due to the extreme thermal energy concentrated into a very small space with no relief. This damage is often referred to as engine knocking and can lead to permanent engine damage if it occurs frequently.

The octane rating is a measure of the fuel's resistance to self-ignition, by increasing the temperature at which it will self-ignite. A fuel with a greater octane rating allows for a much higher compression ratio, virtually eliminating the risk of damage due to self-ignition.

Diesel engines rely on self-ignition for the engine to function. The premature ignition problem is solved by separately injecting high-pressure fuel into the cylinder shortly before the piston has reached TDC. Air without fuel can be compressed to a very high degree without concern for self-ignition, and the highly pressurized fuel in the fuel injection system cannot ignite without the presence of air.

Octane number, also called Antiknock Rating, measure of the ability of a fuel to resist knocking when ignited in a mixture with air in the cylinder of an internal-combustion engine. The octane number is determined by comparing, under standard conditions, the knock intensity of the fuel with that of blends of two reference fuels: iso-octane, which resists knocking, and heptanes, which knocks readily. The octane number is the percentage by volume of iso-octane in the iso-octane–heptanes mixture that matches the fuel being tested in a standard test engine.

RON and MON Octane Number:

Unleaded fuels carry a RON (Research Octane Number) rating. Put simply, RON determines petrol's 'anti-knock' quality or resistance to pre-ignition; or if you want to put in another way, the Octane Number denotes its resistance to detonation. If you run your vehicle on low octane petrol you might notice a 'knocking', 'rattling', or 'pinging' sound (as it’s often called), which means the fuel is detonating instead of burning smoothly. This is not only a waste of energy, but it can also damage your engine in the long run. Burning is the desired effect of any internal combustion engine (not an explosion per se).

Fuel with a higher octane number suitable for your vehicle's engine will eliminate knocking. Older cars that were designed to run on a lower RON fuel can also benefit from a higher RON, because the older the car and the higher the kilometers, means the engine will have a greater propensity to knock. This is mainly caused by a build-up of contaminants and carbon deposits which, when hot, can cause pre-ignition. Rotary engines suffered from this too. As carbon deposits build up on the three apex seals of each rotor, the deposits get so hot, they glow orange with heat and then bang…detonation!

If you’ve ever seen an apex seal with what looks like burnt, corroded and ‘blown’ corners, you’ll know why. So in effect, a higher RON fuel when used in these situations will have a much higher threshold to detonate, therefore reducing that nasty characteristic of detonation.

FCC Octane MON versus RON

Significance of RON and MON:

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For clarity it is useful to provide a general overview of an octane number. An octane number is a quantitative, but imprecise measure of the maximum compression ratio at which a particular fuel can be utilized in an engine without some of the fuel /air mixture "knocking" or self igniting. This self ignition of the air/fuel mixture in the cylinder results in a loss of peak power. Directionally as the compression ratio of the engine increases so does the required octane number of the gasoline if engine knocking is to be avoided.

The performance of an engine is dependent upon many factors, one of which is the severity of operation. Accordingly the performance of a fuel is also dependent upon engine severity. To account for differences in the performance quality of a fuel two engine octane numbers are routinely used. The Research Octane Number (RON or F1) simulates fuel performance under low severity engine operation. The Motor Octane Number (MON, or F2) simulates more severe operation that might be incurred at high speed or high load. In practice the octane of a gasoline is reported as the average of RON and MON or R+M/2.

Classically, both numbers are measured with a standardized single cylinder, variable compression ratio engine. For both RON and MON, the engine is operated at a constant speed (RPM's) and the compression ratio is increased until the onset of knocking. For RON engine speed is set at 600 rpm and MON is at 900 rpm.

MON and RON Depend on Gasoline Composition:

The octane number measured is not an absolute number but rather a relative value based on accepted standards. By definition, n-heptane has an octane number (RON and MON) of 0, while iso-octane (2,2,4-trimethyl pentane) is 100. Linear combinations of these two components are used to measure the octane number of a particular fuel. A 90%/10% blend of iso-octane/n-heptane has an octane value of 90. Any fuel knocking at the same compression ratio as this mixture is said to have an octane number of 90.

In general, RON values are never less than MON, although exceptions to this rule exist. For pure compounds the differences between RON and MON range from 0 to more than 15 numbers. Typical values for gasoline range hydrocarbons having boiling points between 30° and 350° F go from less than 0 to greater than 100 with the extreme values being generated by extrapolation. Table 1 summarizes actual RON and MON values for a variety of pure hydrocarbons.

In practice octane numbers do not blend linearly. To accommodate this, complex blending calculations employing blending octane numbers as opposed to the values for pure hydrocarbons are routinely employed. There is no universal blending program used industry wide. In fact, for a given oil company, blending calculations that are refinery specific are not uncommon. As an improvement over octane numbers of pure compounds, there are tabulations of blending octane numbers for both RON and MON. Summarized in Table 1, these numbers are measured by blending 20 vol.% of the specific hydrocarbon in 80 vol.% of a 60/40 iso-octane/n-heptanes mixture. Although still not exactly indicative of the actual blending octane number for a specific gasoline composition, the blending octane numbers are more representative. In general, the blending octane numbers are greater than the corresponding pure octane number.

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Cetane number:

Cetane number or CN is actually a measure of a fuel's ignition delay; the time period between the start of injection and the first identifiable pressure increase during combustion of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils.

To measure the cetane number properly is rather difficult, as it requires burning the fuel in a special, hard-to-find, diesel engine called a Cooperative Fuel Research (CFR) engine, under standard test conditions. The operator of the CFR engine uses a hand-wheel to increase the compression ratio (and therefore the peak pressure within the cylinder) of the engine until the time between fuel injection and ignition is 2.407ms. The resulting cetane number is then calculated by determining which mixture of cetane (hexadecane) and isocetane (2, 2, 4, 4, 6, 8, 8-heptamethylnonane) will result in the same ignition delay.

How Does Cetane Number Affect Engine Operation?

There is no benefit to using a higher cetane number fuel than is specified by the engine's manufacturer. The ASTM Standard Specification for Diesel Fuel Oils (D-975) states, "The cetane number requirements depend on engine design, size, nature of speed and load variations, and on starting and atmospheric conditions. Increase in cetane number over values actually required does not materially improve engine performance. Accordingly, the cetane number specified should be as low as possible to ensure maximum fuel availability." This quote underscores the importance of matching engine cetane requirements with fuel cetane number.

Diesel fuels with cetane number lower than minimum engine requirements can cause rough engine operation. They are more difficult to start, especially in cold weather or at high altitudes. They accelerate lube oil sludge formation. Many low cetane fuels increase engine deposits resulting in more smoke, increased exhaust emissions and greater engine wear.

Using fuels which meet engine operating requirements will improve cold starting, reduce smoke during start-up, improve fuel economy, reduce exhaust emissions, improve engine durability and reduce noise and vibration. These engine fuel requirements are published in the operating manual for each specific engine or vehicle.

Overall fuel quality and performance depend on the ratio of paraffinic and aromatic hydrocarbons, the presence of sulfur, water, bacteria, and other contaminants, and the fuel's resistance to oxidation. The most important measures of fuel quality include API gravity, heat value (BTU content), distillation range and viscosity. Cleanliness and corrosion resistance are also important. For use in cold weather, cloud point and low temperature filter plugging point must receive serious consideration. Cetane number does not measure any of these characteristics.

Variable length intake manifold:

In internal combustion engines, a variable length intake manifold (VLIM), or variable intake manifold (VIM) is an automobile internal combustion engine manifold technology. As the name implies, VLIM/VIM can vary the length of the intake tract - in order to optimize power and torque across the range of engine speed operation, as well as help provide better fuel efficiency. This effect is often achieved by having two separate intake ports,

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each controlled by a valve, that open two different manifolds - one with a short path that operates at full engine load, and another with a significantly longer path that operates at lower load.

There are two main effects of variable intake geometry:

Swirl:

Variable geometry can create a beneficial air swirl pattern, or turbulence in the combustion chamber. The swirling helps distribute the fuel and form a homogeneous air-fuel mixture - this aids the initiation of the combustion process, helps minimize engine knocking, and helps facilitate complete combustion. At low revolutions per minute (rpm), the speed of the airflow is increased by directing the air through a longer path with limited capacity (i.e., cross-sectional area) - and this assists in improving low engine speed torque. At high rpms, the shorter and larger path opens when the load increases, so that a greater amount of air with least resistance can enter the chamber - this helps maximize 'top-end' power. In double overhead camshaft (DOHC) designs, the air paths may sometimes be connected to separate intake valves so the shorter path can be excluded by de-activating the intake valve itself.

Pressurization:

A tuned intake path can have a light pressurizing effect similar to a low-pressure supercharger - due to Helmholtz resonance. However, this effect occurs only over a narrow engine speed band. A variable intake can create two or more pressurized "hot spots", increasing engine output. When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.

Pressure wave supercharger:

A pressure wave supercharger is a type of supercharger technology that harnesses the pressure waves produced by an internal combustion engine exhaust gas pulses to compress the intake air. Its automotive use is not widespread; the most widely used example is the Comprex, developed by Brown Boveri. Ferrari tested such a device during the development of the 126C Formula One car. The system did not lend itself to as tidy an installation as the alternative twin-turbocharger layout, and the car was never raced in this form[2]. A more successful application was in the RF series diesel engine found in the 1988 Mazda 626 Capella; ultimately 150,000 Mazda diesel cars were fitted with a Comprex supercharger.

The process is controlled by a cylindrical cell rotor whose speed is synchronized with the engine crankshaft speed via a belt or chain. Individual cells alternately open and close the exhaust gas and fresh air apertures, when the aperture on the exhaust gas side is reached pressurized exhaust gas flows into the cell and compresses the fresh air there. As the cell rotor continues to rotate and reaches the aperture on the inlet side the compressed air flows to the engine. Before the exhaust gas can flow the aperture is

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closed again and the exhaust gas column is reflected before entering the engine. The exhaust gas exits at high speed sucking further intake air into the cell behind it repeating the process.

First air is drawn into the cylinder like any other internal combustion engine by the downward movement of the piston, and with the Comprex in its path the air naturally must pass through it first. Inside the compressor air is turned and the blades are moved past the ports on either end of the chamber. When hot exhaust gases enter the chamber it is at a much higher pressure than that of the air that is already in the Comprex which in turn starts a pressure equalization process. When two compressible mediums change state they change by means of pressure waves. So when each blade passes by the exhaust inlet port air enters that cell (the area between each of the blades) and sends a pressure wave towards the intake air at the speed of sound. Now for the tricky part. Since the wheel is turning perpendicular to the movement of the pressure waves and with the help of physics the waves then move in a slanting motion towards the other end of cell, compressing the intake air (which is at atmospheric pressure) to the pressure level of the expanding exhaust gases. The exhaust gas then follows the pressure wave at a much lower velocity.

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