HCCI, Lean Burn

Homogeneous Charge Compression Ignition

(HCCI) Engines

What is an HCCI Engine?• HCCI is a form of internal combustion in which

the fuel and air are compressed to the point of auto ignition.

• That means no spark is required to ignite the fuel/air mixture

• Creates the same amount of power as a traditional engine, but uses less fuel. Traditional combustion

(left) uses a spark to ignite the mixture. HCCI (right) uses piston compression for a more complete ignition.

How Does It Work?• A given concentration of fuel and air will spontaneously ignite when it

reaches its auto-ignition temperature.

• Its main characteristic is that the ignition occurs at several places. This makes the fuel –air mixture burn nearly simultaneously.

Contd….

• The concentration/temperature can be controlled several ways:• High compression ratio• Preheating of induction gases

• The real challenge is to control the combustion.

Advantages• Can achieve up to 15% fuel savings• Lower peak temperature leads to cleaner

combustion/lower emissions• Can use gasoline, diesel, or most alternative

fuels

HCCI automobiles could reduce greenhouse gas emissions

Disadvantages• Higher cylinder peak pressures may

damage the engine• Auto-ignition is difficult to control• HCCI Engines have a smaller power range

• RCCI are also being developed.

Prototype HCCI car from Saturn

The Future of HCCI• The future of HCCI looks promising• Major companies such as GM, Mercedes-

Benz, Honda, and Volkswagen have invested in HCCI research.

Controlling HCCI

• Controlling combustion in HCCI is a major problem.• Variable compression ratio• Variable induction temperature• Variable exhaust gas percentage• Variable valve actuation• Variable fuel ignition quality

Lean Burn Engines

What are they

• lean amount of fuel supplied to and burned in an engine’s combustion chamber.

• Gasoline burns best in internal combustion engines when it is mixed with air in the proportions of 14.7:1.A true lean-burn can go as high as 32:1.

Why are they required

• In order to reduce these harmful exhaust emissions, two basic approaches have been used-

• Catalytic convertors that clean up the exhaust gases coming from the engine• lean-burn engines which produce lower levels of emissions by better

combustion control and more complete fuel burning inside the engine cylinders.

Engineers have known for years that a leaner air to fuel mixture is a frugal engine.

The problems are, if the mixture is too lean, the engine will fail to combust, and a lower fuel concentration leads to less output.

Process in Lean Burn Engines

• Lean-burn engines overcome these issues by employing a highly efficient mixing process.

• Special shaped pistons are used.Additionally, the engine’s inlet ports can be shaped to cause “swirl” — a technique borrowed from direct injection diesel engines.

Advantages

• Very economic (both gasoline and diesel)

• Lean burn engine also have advantage of decreasing emissions. The amount of CO emitted is less as plenty of oxygen is available to produce CO2.

• Unburned HC emission is reduced by upto 80% in lean burn engines.

• The downside of lean-burn technology is increased exhaust NOx emissions (due to higher heat and cylinder pressure).

• So vehicle using them require more complex catalytic convertors.

Stratified Charge Engine

Applications

Advantages

Disadvantages

Catalytic Convertor

•  Modern vehicles try to keep the air-to-fuel ratio very close to the stoichiometric point.

•Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1.

•The fuel mixture actually varies from the ideal ratio quite a bit during driving, and it can be either lean or rich.

•The main emissions of a car engine are:-• N2

• CO2

• H2O

•  But because the combustion process is never perfect, amounts of harmful emissions are also produced in car engines. 

•Catalytic converters are designed to reduce all three-

• Carbon monoxide• Hydrocarbons or volatile organic compounds• Nitrogen oxides (NO and NO2, together called NOx)

How Catalytic Converters Reduce Pollution

• In the catalytic converter, there are two different types of catalyst at work, a reduction  catalyst and an oxidation  catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium.

• The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream.

• In modern day vehicles we have three- way catalytic convertors because they are used to reduce the concentration of three main pollutants.

Superchargers & Turbochargers

• Supercharging is a practice in IC engine for improving engine power output.

• The power output of naturally aspirated engines depends mainly on amount of air inducted per unit time and degree of utilization of this air

• Three possible methods utilized to increase the air consumption of an engine are as follows:

• Increasing piston displacement• Running the engine at higher speeds (number of power strokes)• Increasing the density of the charge

• The method of increasing the air capacity of an engine is known as supercharging. The device used to increase the air density is known as supercharger.

• Supercharger is merely a blower or a compressor that provides a denser charge to the engine.

IN SI engines

• Supercharging in SI engine is employed only in aircraft and racing car engines. Apart from increasing the volumetric efficiency of the engine, supercharging results in an increase in the intake temperature of the engine.

• This reduces the ignition delay and increases the flame speed. Both these effects result in a greater tendency to knock or preignite.

• For this reason, the supercharged petrol engines employ lower compression ratios.

In CI engines

• In case of CI engines, supercharging does not result in any combustion problem, rather it improves combustion.

• Increase of pressure and temperature of the inducted air reduces ignition delay, and hence the rate of pressure rise results in a better, quieter and smoother combustion.

Types of supercharger

• Centrifugal type supercharger• Root’s supercharger• Vane type supercharger

Turbocharging

Turbocharger: The blower/compressor and the turbine are mounted on the same shaft. The compressor is run by the turbine, and the turbine, in turn, is run by the exhaust gases.

EGR

• The purpose of EGR is to lower the combustion chamber temperature. The temperature must be controlled for two reasons:

• High temp. increases amount of NOx

• High combustion temperatures and pressure cause detonation.

• EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders.

• This dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. NOx is produced in a narrow band of high cylinder temperatures and pressures.

• Amount of EGR is controlled by Engine Management System.

• No EGR is used when maximum power is desired

Works Cited

• “Homogeneous Charge Compression Ignition” http://en.wikipedia.org/wiki/Homogeneous_Charge_Compression_Ignition November, 2008.

• “New HCCI Engine” http://videos.howstuffworks.com/multivu/3284-new-hcci-engine-video.htm November, 2008

• “GM Takes New Combustion Technology Out of the Lab and Onto the Road” http://www.gm.com/experience/fuel_economy/news/2007/adv_engines/new-combustion-technology-082707.jsp