Not sure if this should go in Lounge or Technical, but since I'm a newbie i thought i would be safer in here. I can't say I know alot about cars so if anyone can help me out with this i'd be very appreciative.
Situation: coasting down a hill.
Question: Which uses less fuel? (for a fuel injected car)
1) car in neutral – most people would agree that you use bare minimal fuel whilst idling.
Or
2) car in gear with no throttle application? (in other words are the injectors still firing upon engaged motor deceleration with no throttle application.)
My opinion is I think that the wheels are connected to the engine and are therefore keeping the engine turning over and preventing it from stalling. And therefore you don’t need to use fuel and the ECU cuts . Where as if you’re idling you need to pump in fuel and ignite it to keep the engine turning and prevent stalling.
A friend has said :
“If you're in neutral, the fuel pump is pumping some minimum amount of fuel into the engine (i.e. the throttle is open some minimum amount). If you're in gear and rolling down the hill without your foot on the accelerator, the throttle is still open the same amount as if you were in neutral. The engine is only spinning faster by virtue of the car's weight in accelerating down hill.”
If this is the case then wouldn’t coasting downhill in gear at say 4000rpm be using 4 times as much fuel as when idling?
Another friend said:
“Research will show the momentum of the wheels keep pushing the engine along. A car only stalls if the engine stops because theres not enough energy to move the pistons in the engine. i.e uphill, not enough accelerator, slow speeds with high gear etc. when you need the power created by the ignition of petrol. if you are going at such high speeds, the energy isn't required. Even tho the fuel pump may be pumping, its to keep the fuel line presurised so fuel is there when you need it and not necessarily going into the engine as it should be controlled by the injectors. may be different in a carby car tho”
Fuel pump keep pumping to keep the lines pressurised but no fuel going anywhere? Wouldn’t pressure keep increasing (unless there is some outlet) if that was the case?
Some people have said that the ecu shuts of the fuel injection when decelerating until the car reaches idle … is this complete rubbish? One person says that :
“I'm looking at my OBD training manals for Hondas, and for MPFI and DPFI systems, it says "if the ECM receives a closed throttle input (under 0.5V), the engine speed is above 1100 RPM, and the engine is warmed, the fuel injectors will be shut off."”
This is in line with what I originally thought, but if this is the case then I thought (to throw a spanner in), won’t the catalyst cool off too quickly due to unheated air flowing through it and therefore emissions standards won’t be reached?
Originally Posted by saxman
Bookmarks