A theoretical question (about gasoline engines)

by **AlexG** » Tue May 13, 2014 7:27 pm

Lets say we have a fuel injected ecu controlled, gasoline engine.
When we cruise it gets all the time 14.7 AFR.
If we increase ignition timing (and we don't touch the fuel PT map) at part throttle, the mixture gets slightly leaner, so the O2 sensor tends to enrich the mixture so it gets back to 14.7.
The result, is a worst fuel economy.

Please correct me if i am wrong.

I would be grateful, if you could share your ideas.

Thank you

by **djwacik** » Tue May 13, 2014 10:30 pm

AlexG wrote:Lets say we have a fuel injected ecu controlled, gasoline engine.
When we cruise it gets all the time 14.7 AFR.
If we increase ignition timing (and we don't touch the fuel PT map) at part throttle, the mixture gets slightly leaner, so the O2 sensor tends to enrich the mixture so it gets back to 14.7.
The result, is a worst fuel economy.

Please correct me if i am wrong.

Most of the time if you advance ignition you will get better efficiency so fuel economy should be better. It could get worse only if:
- ignition is over advanced (you are losing energy)
- you reached knock

by **AlexG** » Wed May 14, 2014 3:11 pm

djwacik wrote:
AlexG wrote:Lets say we have a fuel injected ecu controlled, gasoline engine.
When we cruise it gets all the time 14.7 AFR.
If we increase ignition timing (and we don't touch the fuel PT map) at part throttle, the mixture gets slightly leaner, so the O2 sensor tends to enrich the mixture so it gets back to 14.7.
The result, is a worst fuel economy.

Please correct me if i am wrong.

Most of the time if you advance ignition you will get better efficiency so fuel economy should be better. It could get worse only if:
- ignition is over advanced (you are losing energy)
- you reached knock

Thank you very much djwacik for the reply,

I have noticed if i increase ignition timing at WOT map and i don't touch the WOT fuel map the AFR is leaner. Why?

by **techmotor** » Sun May 25, 2014 3:40 am

Hi AlexG

Timing ignition is about how many time you will have for the air/fuel burn to generate biggest pressure after some amount of camshaft degrees.

For example, let´s mark 15º degress ATDC (After top dead center) for the mixture check point

If you use a amount of advance timing let´s say, 9º BTDC (Before top dead center) you will have less time to burn "all" the amount of fuel than 12º BTDC.

So you have more time to burn the mixture before 15º check point, that´s is more efficiency, and consequently a littler more leaner mixture.

Your engine was able to use better all chemical inside the engine, like O2, Fuel, N2, H2, etc

According the burned amount of Oxigen at exhaust ECU will read as leaner, richer, etc.