Thread: Why suck more fuel under load

But the answer is simple...
When towing or going up a hill, to keep up speed you have to put your foot down, opening up the intake butterfly and increasing the airflow, and in an injected car also affecting the injection via the ecu. So the revs may stay the same but the amount of fuel and air has to increase so that energy out=energy in or the car would slow and stall.
Dave

Gravity. going up hill you are working against gravity and so putting more drag on the car. Going down hill you are working with gravity and so less drag. More drag=more fuel to overcome drag. Would you rather push a car up hill or down? Would you sweat more pushing a car up hill or down? To keep a constant RPM with a greater load you must increase the amount of torque, more torque=more fuel.

The laws of thermodynamics, and simple physics suggests that effort and power applied, and energy consumed during the process are all intertwined.

This is why you may puff a little harder walking up stairs....

JC

Geez mate if you had someones hands around your throat choking you, and then they took their hands away, you would suck in more air.

Petrol engines are throttled by the throttle butterfly at all conditions but full throttle. This is what causes "manifold vacuum". The pistons want to suck more but they cannot.

The computer or carby ensures that there is a combustable fuel air mix at all concentrations of fuel/air going into the engine. At cruise, the fuel /air mix is also leaner, maybe 1 part of fuel to 14.7 parts air, as it is unlikely that there will be detonation.
When you put your foot down going up a hill, the throttle lets in more air plus fuel to keep the same proportions, or maybe more fuel compared to air( 1 part fuel to 13 parts air) to keep the cylinder cool and prevent detonation.
It's not so much the hill but your foot on the accelerator that causes more fuel use. If you left your foot in the same position and slowed down on the hill you would use the same mount of fuel as going along the straight. (within reason, this depends on the rev point of Brake Mean Effective Pressure BMEP of the engine)

The above is one reson why petrol engines use more fuel than diesels.
Diesels do not throttle the air. The difference in power between cruise and full power is from the different ratio of fuel to air that the pump delivers as you push down on the accelerator. Diesel also has more energy per litre than petrol.( I put that in because someone is sure to , if I didn't)

Recent advances in petrol engines are bringing them much closer to diesels in economy at a much cheaper cost of production.
The main ones are
Direct injection enabling more accurate injection timing.( VW, Mitsubishi)
Elimination of throttle by using camshaft timing. (BMW engines)
Compound supercharging (VW Golf GT 1400 CC with supercharger and turbocharger , and direct injection )
Regards Philip A

OK a simple one....

now stop embarrasing me any further

Vacuum is a low pressure void in the cylinder and manifold vacuum is controlled by the throttle butterfly but an engine doesnt suck fuel in, it is either forced through the jets by atmospheric pressure or pumped in by the accelerator pump or the injector HP pump.

Just thought Id clarify that as it was something that my tradeschool fuels teacher was a hard a$$ about.

But as everyone else has said, gravity and load agianst speed and Accelerator pedal/throttle butterfly position.

As phillipA said, open the throttle butterfly and more fuel goes in and more power comes out.

Thats about it.