High Pressure Turbocharging On Gas Engines E. Codan S. Vögelih, C. Mathey ABB Turbo Systems Ltd Bruggerstrasse 71a, CH-5401 Baden, Schweiz Abstract High pressure turbocharging opens new development potential for diesel and gas engines. This paper describes improvements for gas engine performance and efficiency while at the same time keeping current low emission values as a priority. The specific problem of controlling gas engine power via mixture mass and equivalence ratio is discussed in detail, taking into account the increased complexity of 2-stage turbocharging. In order to achieve the demonstrated engine performance and efficiency potential, suitable turbocharging concepts are a prerequisite. However, it is of utmost importance that the part- ners involved (i.e. the engine builders and the turbocharger manufacturers) maintain close cooperation in order to realise overall system optimisation. Key Words: Miller Cycle, Gas Engines, 2-stage Turbocharging, Engine Efficiency
20
Embed
High Pressure Turbocharging - ABB Group · PDF fileHigh pressure turbocharging opens new development potential for diesel and gas engines. ... Perfect air Ideal gas ... Gas expansion
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
High Pressure Turbocharging
On Gas Engines
E. Codan S. Vögelih, C. Mathey
ABB Turbo Systems Ltd Bruggerstrasse 71a, CH-5401 Baden, Schweiz
Abstract High pressure turbocharging opens new development potential for diesel and gas engines. This paper describes improvements for gas engine performance and efficiency while at the same time keeping current low emission values as a priority. The specific problem of controlling gas engine power via mixture mass and equivalence ratio is discussed in detail, taking into account the increased complexity of 2-stage turbocharging. In order to achieve the demonstrated engine performance and efficiency potential, suitable turbocharging concepts are a prerequisite. However, it is of utmost importance that the part-ners involved (i.e. the engine builders and the turbocharger manufacturers) maintain close cooperation in order to realise overall system optimisation. Key Words: Miller Cycle, Gas Engines, 2-stage Turbocharging, Engine Efficiency
2
1 Introduction
The market for gas engines is undergoing expansion at a previously unknown rate, especially
in developed countries.
For this there are various political and economic reasons. The availability of the fuel, involv-
ing both the material itself and the supply infrastructure, is a basic precondition. The pricing
policies of the individual countries based, in part, on considerations regarding storage, avail-
ability and dependence on producers, play an equally important role.
There are, however, also technical reasons for the success of the gas engine. Technological
progress in the combustion as well as in the process of gas engines has already brought them
to a point where power density and efficiency can be compared with the values of a diesel
engine.
As a third factor emissions behaviour plays an important role. By their nature gaseous fuels
allow combustion at low noxious emissions values. In particular, particulate emissions are at a
very low level. Good engine efficiency and the favourable hydrogen to carbon ratio of the fuel
guarantee an advantage regarding CO2 emissions. By means of lean burn technology, NOx
emissions can be held at an extremely low level without appreciable penalties in terms of en-
gine efficiency. In this way, the opportunity arises for gas engines to achieve significantly
better efficiencies than diesel engines of the same output while complying with the coming
round of tightened emissions limits.
As with diesel engines, high pressure turbocharging will make an important contribution in
the further progress of gas engines. In this report ways will be examined by which the full
potential of gas engines can be exploited using high pressure turbocharging.
ABB Turbo Systems Ltd (ABB) is making its own contribution to the improvement of the
performance and emissions behaviour of gas engines through its own studies, close coopera-
tion with leading manufacturers of gas engines and by making available suitable products.
3
2 A Short History of the Gas Engine
In the area of large engines, since the 1930’s many diesel engines were also offered in gas
engine versions for specific applications. The engines concerned were, in part, pure gas en-
gines with stoichiometric combustion and spark plugs which, with turbocharging, were devel-
oped to mean effective pressure levels of 10 to 12 bar. Efficiencies were lower than with the
corresponding diesel engines but higher than those of petrol engines. This was thanks to the
influence of size, improved knock behaviour (methane has an octane number of about 130)
and configuration for stationary operation with lower throttle losses. The main advantages
were the cost and availability of the fuel, clean combustion and the possibility of recovering a
great deal of heat from the hot exhaust gases.
Parallel to this development, engines in the "dual fuel" category were developed which can be
used in both gaseous and liquid fuel modes. The fuel injection system is configured for two
operating modes: pure diesel operation up to full load and gas operation with, typically, injec-
tion of 3 to 7% of diesel fuel as an ignition pilot. The operating values of these engines lie
between those of diesel and gas engines. In order to guarantee both knock-free operation and
flammability of the diesel fuel compression ratios lie in the area of 11. The power density of
dual-fuel engines has, in the meantime reached values of up to 20 bar mean effective pressure
(pme).
With pure gas engines significant progress was achieved in the 1990’s via the development of
new combustion technologies, especially for lean burn operation. In this way mean effective
pressures were raised to 14 to 16 bar in combination with improved efficiency. Today’s en-
gines profit from the introduction of the Miller process. This allows mean effective pressures
of over 20 bar to be achieved. At the same time engine efficiencies are similar to equivalent
diesel engines and in some cases even higher.
The content of this report is the further development of the gas engine up to extreme Miller
valve timings under the application of high pressure turbocharging. In this way it will be pos-
sible to further increase performance and eliminate the deficits versus diesel engines even in
the area of power density.
2.1 Ignition System (Fig 1)
Classic, spark ignited Otto cycle ignition is not limitlessly scalable. The ignition energy and
the durability of the spark plug become insufficient as engine size increases. It was, however,
possible to widen the application range of spark plugs by the use of pre-chambers. The pre-
chamber can be supplied with additional gas in order to further improve the conditions for
combustion (pre-chamber mixture enrichment).
4
Ignition using diesel injection still remains a valuable alternative for large engines. Common
rail technology has allowed a reduction in pilot injection to below 1% of the total energy (Mi-
cro pilot injection). Diesel injection can take place both in an open combustion chamber or a
pre-chamber.
2.2 Gas Admission
For spark ignited (Otto) engines using liquid fuels, the development of the carburettor has
lead to indirect injection and then direct injection into the combustion chamber. In gas en-
gines, by contrast, the energy needed for compressing the fuel plays a much more significant
role. In the extreme case it can involve up to 15% of engine power. For this reason small gas
engines, above all, are operated with a central atmospheric gas mixer. This also allows eco-
nomic operation on weak gases which need to be mixed with the air in not inconsiderable
quantities.
Gas admission in the inlet port, generally using timed dosing valves, is widespread on large
engines. This has the advantage that gas exchange and power control can be achieved in a
similar way to diesel engines. Scavenging of the combustion chamber with air is possible and
the variation in gas quantity within the permissible λv fluctuation tolerance can be rapidly
achieved.
Direct injection into the combustion chamber is very rare on gas engines. An example for this
class is represented by so-called diesel-gas engines. In this category diesel and gas are in-
jected simultaneously, which can be achieved via two injectors or an injector with two con-
centric nozzle rings. These engines should be classed as genuine diesel engines with every
advantage and disadvantage. However, the handling of gas at high pressure (250 bar) repre-
sents a further challenge.
Spark ignition
Gas Scavenged Pre-chamber,
Spark Ignition
Pilot Fuel Ignition
.
Figure 1: Possible ignition systems for gas engines
5
2.3 The Layout Diagram
The mean effective pressures achievable on
gas engines is limited by the knock limit
and the stability limit for very weak mix-
tures, (Figure 2).These two boundary areas
leave a free area in which both mean effec-
tive pressure and air:fuel ratio can be in-
creased. This is the area occupied by all
contemporary gas engines in which high
efficiencies and low NOx emissions are
possible. Via the use of the Miller Process
it is possible to shift the knock limit further
upwards and thus further increase mean
effective pressure and engine efficiency.
3 Thermodynamic Principles
It has already been demonstrated [2] that
the Miller Process can make a large contri-
bution towards increasing the efficiency of
combustion machines via temperature re-
duction. The analysis of the working proc-
ess with perfect air does not help in deter-
mining the achievable potential (Figure 3).
Only the red curves in the diagram were
calculated using perfect air, for further
curves the working process was calculated
using gas properties reflecting the current
state-of-the-art [3]. For combustion air and
a reference, hydrocarbon based fuel the temperature influence of the chemical species was
taken into account (ideal gas model). The influence of pressure (real gas model), as well as
dissociation at high gas temperatures – both of which would further reduce process efficiency
- were not taken into account. These influences are considered negligible.
While the black curve is based on a conventional process, for the green curves a Miller Proc-
ess having an in-cylinder expansion ratio of 2 was assumed. This reduces the starting tem-
perature from 80°C to 17°C.
Figure 2: Influence of excess air ratio (λv) on per-
formance, emissions and limits of the gas engine
[1].
0.54
0.56
0.58
0.60
0.62
0.64
0.66
0.68
0.70
150 200 250 300 350Pmax [bar]
ηηηη t
Perfect air
Ideal gas
Ideal gas, Miller
εεεε = 16, λλλλ V = 2.2, pmi = 30 bar
εεεε = 14, λλλλ V = 2.1, pmi = 30 bar
Figure 3: Ideal cycle thermal efficiency compari-
son.
6
The diagram shows that the efficiency level increases by 2 to 3 percentage points. The ideal
Miller Process may result in a loss of around 1.5 % but this is more than compensated by an
increase of up to 5 % due to the temperature reduction. This considerable gain results from
two roughly equal contributions: the more favourable high pressure portion and a gas ex-
change loop with a larger positive area. The first set of curves (solid lines) refers more to die-
sel engines with a high compression ratio ε. For gas engines it is to be expected that the ε-area
will be located in the range 13 to 15 (dashed curve). The air:fuel ratio is only slightly smaller
than for a diesel engine.
3.1 Gas Exchange
The reason why the process efficiency can be greatly increased via the gas exchange phase is
explained in Figure 4. The p-V diagram on the left shows the idealised gas exchange process
with conventional turbocharging. An increase in charging efficiency from 65% to 75% gives
the possibility of slightly increasing efficiency, using more piston work in the gas exchange
phase. As an alternative the pressure difference over the engine can be left unchanged result-
ing in more energy being exploited from a turbine (turbocompounding).