A serious thread- for once

W

Woots

Veteran
 Milton Keynes
Got a question, bear with me here...although im an aircraft engineer i will admit im not fully up to speed on car mechanics, so this might seem stupid but its worth an ask...here we go

Imagine you are in a car...going up a hill...in 6th gear in a regular car
Now obviously depending on how far you push the throttle down results in a proportional amount of fuel/air being forced into the engine (assuming the throttle operates a butterfly valve) now the engine may not rev much more due to the fact you are in too high a gear for the revs to build quickly...so say the engine rpm goes from 2000-2500 rpm...not much of an increase but due to WOT being applied more fuel will be burnt...

However, say the same car next time around decides instead to drop down to 3rd and apply less throttle....i duno say 40-50% throttle....being a lower gear the revs now rise faster.....less throttle is applied so less fuel is needed (in a way) but obviously due to the higher RPM the engine will then burn more fuel to rev the engine at a greater speed

So my question...which would burn less fuel.....
WOT in a high gear and low rpm
Or less throttle in a lower gear and higher rpm
Might seem dead obvious....and my head is saying the higher rpm outweighs the reduction of throttle openage...but just curious...
Cheers
 
i would say lower rpm less fuel .the same amount of fuel will enter when the inlet valves are open so more times the valves are open = more fuel.
 
Lower gear more throttle is best BUT if you used your training and installed heli blades for such circumstances then neither would be necessary.
 
Adamski said:
So Woots, forget this serious car chat. Blondes or brunettes? :)


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HAHAHA
Im trying to be sensible and you guys are INSISTENT on making this forum a crude and deviant environment
Love it!!





Oh and blondes :lol:
 
But lads - lower gear takes longer to accelerate, so you'll be putting the engine under load for longer before easing off. Could burn more petrol?

Devils advocate....


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I have wondered similar

there is more friction at higher revs so that isn't going to help
sometimes cars get better economy with a larger engine.


A guess but I suspect dropping a cog is less efficient.
We need an expert really.
 
If you go the other way, down the hill, it will use less fuel.
 
I think more revs, less throttle. Only because I noticed it on my sisters Clio on the computer that its more economical to keep it revving slightly longer than to change early and labour the engine.

Having said that, newer engines, especially TD engines with the correct gear change options thing seems to get you to change up very early!
 
Adamski said:
But lads - lower gear takes longer to accelerate, so you'll be putting the engine under load for longer before easing off. Could burn more petrol?

Devils advocate....


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Assume that due to the lower throttle openage the speed both times rises an identical amount.....
 
Without going into maths and physics, both of which I hate.... it's hard to tell. Would depend on the engine load, speed and also the hill gradient.

Labouring the engine is not good. Putting the engine under load if it isn't accelerating is also not good.

How steep is the hill Woots?


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Its all about the bang , large displacement engines lets say a big 6.0ltr V8 would probabley go up a hill in top gear on tic over due to the huge amount of torqe( the large bang) a small displacement engine lets say a 1.0 ltr does not have the big bang but a little bang , (so not much power) so if the small engine car is in a top gear the engine will struggle to turn the crank thus drive the wheels this will result in using a lower gear to get up a hill .

Its like a bicycle gearing, large cog drives small cog needs more leg power to turn wheels but results in higher speeds, small cog drives large cog less leg power lower speed. So leg power = fuel . Hope this sort of makes sence :?
 
Engines have a certain volumetric efficiency at certain rpms. I think it's all down to where in the rev range your torque and HP meet. That is the most efficient point. And remember. When in 3rd as opposed to 6th. The torque multiplication is greater. and the engine will not have to work so hard. It's why when I owned a huge 16 litre 550HP 1850lbft/2500newton-meters torque caterpillar engined Kenworth. Say that quickly three times! I still needed 18 forward gears to keep it in the proper rev range. To high numerically of a gear or to low an rpm for the hill and the engine exhaust gas temp would skyrocket to 1400 deg F if I kept my foot on the bean pedal.

Of course I could be full of beans.
 
hmmm seems its not that obvious with ppl giving different reasons
i think a practical experiment needs to be carried out....
 
If your foot is flat on the accelerator to maintain speed, it'll burn more fuel than a lower gear and higher RPM


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