Engine Theory Question
Discussion
you have maintained engine speed but increased load.
therefore you will use more fuel (ceteris parabus).
or another way of looking at it....
at a constant engine speed, if you have opened the throttle you are ingesting more air, and more fuel to go with it (as the air/fuel ratio won't vary much - if anything it will get richer).
N
therefore you will use more fuel (ceteris parabus).
or another way of looking at it....
at a constant engine speed, if you have opened the throttle you are ingesting more air, and more fuel to go with it (as the air/fuel ratio won't vary much - if anything it will get richer).
N
When you open the throttle on an engine to go up a hill, but maintain constant speed and revs, does it use more fuel?
If so, how?
Yes. When you open the throttle more air and fuel goes into each cylinder per revolution, producing more pressure, and hence more torque and power (at the same speed).
If you've got constant revs, then the same volume of mixture is being pumped the whole time. To use more fuel you've got to either make the mixture richer, or more dense?
A friend of mine reckons the extra power comes from increased pumping efficiency alone, but I can't help but think that it must use more fuel too?
A friend of mine reckons the extra power comes from increased pumping efficiency alone, but I can't help but think that it must use more fuel too?
It's not quite the same as a tap, cos it's not going from a closed system (pipe) to an open system (the air).
I reckon that although the mixture density in the inlet tract doesn't change, the final mixture density in the combustion chamber before the inlet valve closes is higher, because more fuel and air have been allowed in.
Sound reasonable???
I reckon that although the mixture density in the inlet tract doesn't change, the final mixture density in the combustion chamber before the inlet valve closes is higher, because more fuel and air have been allowed in.
Sound reasonable???
In simple terms it takes more effort to lift a weight than to push it on the horizontal, so add the extra effort to move the car uphill to the effort required to maintain speed on the flat and you are working harder. For an engine to work harder it needs more fuel (and air) even at the same revolutions per minute
I think that makes sense
Edited to add:
and Matt - yes you can! (by increasing the effort required, e.g. go uphill, reduce aerodynamic efficiency by deploying an air brake etc.
Edited again to add:
On my Range Rover travelling on a motorway at a fixed throttle opening the car speed would vary by about 5 mph dependent on the aircon being busy or not. To maintain a constant speed the engine had to work harder when the aircon was busy so more throttle was required, the aircon cycled several times a minute, thank goodness for the cruise control on my later one!
>> Edited by kevinday on Tuesday 7th January 14:50
>> Edited by kevinday on Tuesday 7th January 14:52
I think that makes sense
Edited to add:
and Matt - yes you can! (by increasing the effort required, e.g. go uphill, reduce aerodynamic efficiency by deploying an air brake etc.
Edited again to add:
On my Range Rover travelling on a motorway at a fixed throttle opening the car speed would vary by about 5 mph dependent on the aircon being busy or not. To maintain a constant speed the engine had to work harder when the aircon was busy so more throttle was required, the aircon cycled several times a minute, thank goodness for the cruise control on my later one!
>> Edited by kevinday on Tuesday 7th January 14:50
>> Edited by kevinday on Tuesday 7th January 14:52
kevinday said: In simple terms it takes more effort to lift a weight than to push it on the horizontal, so add the extra effort to move the car uphill to the effort required to maintain speed on the flat and you are working harder. For an engine to work harder it needs more fuel (and air) even at the same revolutions per minute
I think that makes sense
You can get more energy out of the engine without using any more fuel, simply by making it more efficient. Imagine you're pushing a car and the handbrake is slightly on. You can push it faster without any additional effort by releasing the brake. In this analogy the car is the engine and the brake is the throttle
...I hope that make sense!
Still think that you use more fuel - for the reasons stated earlier.
Hi all,
Today's PH 'Greasy Finger' Award for correctly answering an fundamental Auto Related Question goes to NDT and AJL!
Given the hypothetical question posed by DanHay, NDT is correct that the engine is operating under increased load and AJLintern recognised that you can maintain the same revs, but had to, at some point, open the thottle to accomplish this. Using more fuel was implicit in each of their answers. Good Job Guys!!...Jim '85TE
Today's PH 'Greasy Finger' Award for correctly answering an fundamental Auto Related Question goes to NDT and AJL!
Given the hypothetical question posed by DanHay, NDT is correct that the engine is operating under increased load and AJLintern recognised that you can maintain the same revs, but had to, at some point, open the thottle to accomplish this. Using more fuel was implicit in each of their answers. Good Job Guys!!...Jim '85TE
danhay said:
kevinday said: In simple terms it takes more effort to lift a weight than to push it on the horizontal, so add the extra effort to move the car uphill to the effort required to maintain speed on the flat and you are working harder. For an engine to work harder it needs more fuel (and air) even at the same revolutions per minute
I think that makes sense
You can get more energy out of the engine without using any more fuel, simply by making it more efficient. Imagine you're pushing a car and the handbrake is slightly on. You can push it faster without any additional effort by releasing the brake. In this analogy the car is the engine and the brake is the throttle
...I hope that make sense!
Still think that you use more fuel - for the reasons stated earlier.
Dan/kevin,
Yes, you can change the engine's efficiency, but if you change the load when the engine is in a given state, regardless of it's efficiency, it will have to use more energy to maintain that state.
As the famous quote goes: "Captain, I can't change the Law of Physics..." More simply, according to Isaac Newton: A body (read: car) at rest will remain at rest unless acted upon by a force (read: energy converted and directed by the engine) and A body (read: car) in motion will remain in motion unless acted upon by a force (read: Hill/gravity). To overcome the effects of gravity additional and equal opposing force (read: energy converted and directed by the engine) needs to be applied. It really all boils down to Physics Hope this clarifies...Jim '85TE
>> Edited by lotusguy on Tuesday 7th January 16:13
>> Edited by lotusguy on Tuesday 7th January 17:39
danhay said:
AJLintern said: Put simply, when you start to go up hill you will have to open the throttle more to maintain the same revs/speed, hence you are using more fuel.
Yes but can you explain the "hence" a bit more fully?
Essentially, the engine management system measures the amount of air going in to the engine and injects a proportional amount of fuel. Air/fuel ratio is fixed and power is determined by the amount of air drawn in. Reason that the engine takes in a variable amount of air although it has a fixed cylinder size, is that at part throttle it is drawing in air past a restriction (the throttle) which means the cylinders are under partial vacuum.
(Incidentally, diesels are quite different - they don't have a conventional throttle at all but power is varied by changing the air/fuel ratio. And I seem to remember reading about a new Merc engine that was a hybrid between petrol and diesel - no throttle, but still spark ignition using some devious stratified charge.)
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