- #1
suomi123
- 4
- 0
I'm a mechanic who is currently doing a study automotive engineering.
Being someone who worked on both diesel and petrol cars I know what a higher or lower air fuel ratio (AFR) will do for both engines. However I don't fully understand how the following is possible.
"Adding fuel in a diesel engine will result into more heat, however adding fuel in a petrol engine removes heat. Removing fuel from a diesel results into lower temperatures, however removing fuel from a petrol engine increases heat."
It's like saying adding wood (fuel) to a fire will make a bigger fire with more heat, but then also saying adding more oxygen a fire increases heat. Both statements seem true, but how is that possible?!
Here I will sum up some things so they won't have to be mentioned.
For diesels-
Being someone who worked on both diesel and petrol cars I know what a higher or lower air fuel ratio (AFR) will do for both engines. However I don't fully understand how the following is possible.
"Adding fuel in a diesel engine will result into more heat, however adding fuel in a petrol engine removes heat. Removing fuel from a diesel results into lower temperatures, however removing fuel from a petrol engine increases heat."
It's like saying adding wood (fuel) to a fire will make a bigger fire with more heat, but then also saying adding more oxygen a fire increases heat. Both statements seem true, but how is that possible?!
Here I will sum up some things so they won't have to be mentioned.
For diesels-
- Adding fuel in a diesel engine rises EGT's (exhaust gas temperature)
- Diesel engines always run lean (even though their might be rich pockets)
- Diesel engines achieve combustion by pressure and heat
- Diesel engines use their injection system to time their combustion (so no pre-ignition is possible)
- Diesel engines don't have a real throttle valve
- Going rich results into unburned fuel which turns into gas absorbing heat (temperature decreases)
- Leaning out in a petrol engine rises temperatures.
- Since leaning out increases temperatures pre-ignition and knocking might occur
- Petrol engines have to stay within a small range of AFR's
- Petrol engines have a throttle valve limiting air going into the engine
- If we would run a diesel engine very rich, for example with a AFR of 13:1 will have the same cooling effects we see in petrol engines running rich mixture? (ignoring practical problems such as soot and NOx production)
- If we go lean in a petrol engine it will run hot. But what if we go beyond slightly lean, let's just say we run a AFR of 30:1. Would we see a drop in temperatures?
- What is the limiting factor in going lean? Can we simply keep going leaner till the engine starts struggling with combustion? Or is the limiting factor lubrication (from fuel)?