Hi,
To start an engine, you have to initiate combustions which is possible only with a good air fuel mixing rate. This rate depends on the fuel and temperature because they have an influence on vaporisation.
As the temperature gets colder, the vaporisation of the fuel is harder and so the area where air and fuel are enough mixed to start a combustion are harder to find to propagate the flame.
To solve those cold start problems, a little more of fuel is injected to ensure that there is enough fuel mixed with air. The major drawback of this is an increase of pollutant emissions, and because of today stricter emission standards, it is a major problem for engine calibration.
I studied somewhere that in cold days some petrol is mixed with the diesel in diesel engine vehicle to avoid start delay problem in cold weather. is it true? please justify this.
I agree with Avinash that a cold engine block would absorb the heat of compression of the air-fuel mixture, which prevents the mixture from reaching the ignition temperature. In petrol engines, the spark plugs are always hot enough to start combustion.
As for the Diesel-petrol mixture, it's actually used to improve efficiency. The Diesel in the mixture has the role of spark plugs. But unlike the spark plugs that are fixed and produce a spark that has to travel down the combustion chamber, the diesel in the mixture is everywhere and when it ignites, the ignition is sudden and uniform causing the petrol to burn completely and at once.
I am agree with you jose. It may be also a reason for older reason. Thanks for this aspect also..
In fact, the vaporization of the fuel is more difficult if the temperature is lower and so it’s more difficult to start combustion and more difficult to propagate the flame. Thats why to avoid cold start problems, a little more of fuel is injected to ensure that there is enough fuel mixed with air even under this conditions.
This is mainly evaporation problem, where the initial air temperature is very low and so the compressed air temperature will be low as well and it may not be enough for complete evaporation and self ignition and therefore you can find in some Diesel engines, they used a heating plug.
with time the engine body becomes hot and the inlet air temperature increases even the weather is cold.
If I may add some comments on Dr. Elwardany's, although there are many contributors as are stated in this series of comments, the fundamental factor is the difference in fuel properties between gasolene/petrol and diesel fuel/gas oil. The specification standards may vary slightly among countries, there is a crucial difference between gasolene and diesel which is not much different regardless of the standards. It is the distillation point definition. 90% of gasolene/petrol vapourizes below 180 degrees C, while usually the initial distillation point of diesel fuel/gas oil is above 180 degrees C and the 90% vaporization point is ca. 350 degrees C. You can have ready to burn fuel-air mixture much more easily with gasolene than diesel despite of the fact that gasolene/petrol engines have much lower compression ratios. And starting diesel engines in cold weather simply exemplifies this fact to us.
Diesel combustion is typically operated at lean overall air-fuel ratio or excess air combustion. The combustion occurs after vaporized fuel mixes with air, forms a locally rich but combustible mixture, and the proper ignition temperature is reached. The factor play a major role in the diesel combustion process are the inducted air and the injected fuel's atomization; spray penetration, temperature, and chemical characteristics. Sufficient rapid mixing between the injected fuel and the air in the cylinder to complete combustion in the appropriate crank angle and interval close to top center must be achieved (refer to J.B. Heywood, Internal Combustion Engine Fundamental, Ch.10). Diesel fuel (boiling point property - easily vaporize 180-340 deg.C) is heavier than gasoline (BP of 27-225 deg.C and its RVP property - aware of vapor lock in fuel system causing fuel pumping difficulty).
What are the remedies to start heavy duty diesel engine at -15 degree?
What type of cold starting arrangement is required?
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