PRE-CHAMBER COMBUSTION IN SI ENGINES 7 OCTOBER 2022

BY

BHARATH KUMAR.V

ROLLNO:111120025

BTECH MECHANICAL ENGINEERING

1)FACTORS AFFECTING PRE-CHAMBER COMBUSTION:

➢ EFFECT OF THROAT DIAMETER

Some experimental results have raised concern that small crossectional area of the throat may lead to
higher pressure build up due to flow restrictions inside the pre chamber so that an systematic analysis
was conducted by increasing the throat diameter by keeping nozzle length and diameter constant the
result from the analysis have shown that throat diameter has a major effect on pre chamber peak
pressure and also it substantially effect on residence time of the prechamber.so an assessment is needed
as to whether is it preferred to have a low pressure build up and lower residence time or the opposite.

➢ If the geometrical configuration is such that it promotes a longer residence time (scenario 1), the charge
has more time to react, and the mass being ejected from the pre-chamber is likely to be mostly
composed of hot combustion products. On the other hand, with shorter residence time and lower
pressure peak (scenario 2), there is not sufficient time for complete combustion and the composition of
the jets coming from the PC has a considerable amount of intermediate species, such as OH radicals.

➢ In the first scenario, similar to the curve for the throat diameter of 3.3 mm, if mostly hot complete
combustion products are ejected from the pre-chamber, it indicates that the remaining part of the
charge has a similar composition. Once the jets reach the main chamber, the main charge ignites. When
the flow becomes reversed, it back flows into the pre-chamber part of the charge that is mainly close to
the nozzles.

➢ In the second scenario, more likely the blue curve, the short residence time and low peak pressures may
produce jets with compositions rich of intermediate species. Once back flow happens, some of those
intermediate species, along with combustion products and a minor quantity of fresh charge of CH4/air
are pushed back to the pre-chamber and those are accumulated back inside the pre-chamber. Under
propitious conditions, such as high concentration and high temperature, this “pool” of radicals reacts
and may promote a secondary heat release.this may help to explain the wiggly behaviour of the black
and blue curves after the PC downfall pressure event.

➢ In summary, once the pre-chamber charge ignites, the flame initiates, propagates, and starts pushing
some of the fresh air in front of it towards the main chamber. The flame travels and eventually reaches
the nozzle region. Given both scenarios, the main chamber ignition is triggered by a combination of hot
radicals or simply by hot complete combustion products.

BELOW ARE THE GRAPHS WHICH EXPLAIN THE EFFECT OF THROAT DIAMETER ON
PRECHAMBER AND MAINCHAMBER PRESSURE:
2) Effect of cold EGR:

➢ Experimental results have shown that introducing of cold EGR technology on pre chamber
engine has resulted in reduction of NOx by 50% the emission reduction is because of the
reduction in in cylinder temperature because of exhaust gas being recirculated into the
combustion chamber and using 10% EGR have resulted in reduction of UHC by 50% however
exceeding this value and recirculating the exhaust gas has resulted in increase in UHC emissions
The in cylinder pressure declines as the percentage of cold EGR increases because of the
reduction in amount of oxygen participating in combustion process which results in decrease in
in cylinder pressure as the cold EGR increases the incomplete combustion is likely to occur
more and if it continues misfire could happen which results in stopping of engine and reduction
of engine power these results have shown that increase in cold EGR after a certain range engine
performance is reduced further the cold EGR increases the peak pressure concentration
decreases due to delayed ignition.

3) Effect of ignition plug on pre chamber combustion characteristics:

In this experiment the ignition plug was kept at two positions A and B and pressure rise rate vs phase angle curve
is observed and the results have concluded that if the ignition plug is far from the nozzle exit of the pre chamber
unburnt gases is injected before burnt gases and that burnt gases spreads all over the unburnt one this results in a
steeper rise in the heat release rate however if the ignition plug is in the vicinity of the nozzle exit the combustion
gas is injected during the total injection period which is the period when pre chamber pressure exceeds main
chamber pressure which results in a gradual rise in the heat release rate.

4) Effect of nozzle diameter:

In this experiment for a throat diameter of 3.3 mm three different diameter of nozzle are taken and the results have
shown that smaller diameter of nozzles gives higher jet speeds thereby increases turbulence to provide a proper
combustion also small nozzle diameter promote larger residence time therefore high peak pressure of pc and
higher residence time in pc results in complete combustion of products compared to lower residence time in pc
this directly dictates the composition of the jets and after the pressure downfall event in pc is expected to be
parallel lines instead the wiggly behaviour increases with increase in nozzle diameter.

COMPARISON BETWEEN SI AND PRE-CHAMBER SI COMBUSTION:

➢ In pre chamber SI engine the load is controlled by adjusting the air fuel coefficients whereas
in SI engine it is controlled using throttle valve.

➢ Shorter combustion duration and higher peak of HRR are obtained in pre chamber compared
to SI engine due to increase in burning rate by jet effect.

➢ In pre chamber SI the indicated specific fuel consumption is lower at part load compared to SI
engine.

➢ The pressure oscillation in pre chamber is caused by the fast burning rate after flame ejection
while in SI engine it is caused by end side autoignition