A serious thread- for once

Woots said:
tomotomotomo said:
Have a read about brake specific fuel consumption. I think this article does a good job of explaining:

http://autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_112611/article.html
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so this is saying that the lower gear and lower rpm is better economy
as long as the engine isnt at tickover as if it was the engine wouldnt dissipate heat as effectively..
os tht right
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Sounds right. I got most of that article, but my interpretation that I should floor it through 2,000 to 5,000 RPM, changing at the 5,000 mark each time right up to my highest gear and lowest RPM at chosen cruising speed is probably incorrect :D
 
Find a steep hill,reset your dash computer while travelling in top gear to the top.Note the figures you get.
Go back down the hill,turn around.
Go up the steep hill again,reset your dash computer while travelling in what ever gear to the top.Note the figures you get. :thumbsup: :lol:
 
Adamski said:
Labouring the engine is not good. Putting the engine under load if it isn't accelerating is also not good.
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Exactly. Accelerating from low revs in a high gear (and uphill) places great stress on camshaft bearings and the dual mass flywheel.
 
C'mon Woots, what's the real story here ?

Was this a question you got wrong on the final ?


If you take your data from the slow run ( mpg ) and compare it to the data from the high rpm run( mpg x fK ) and you'll see the correct answer.


fK = fun factor coefficient of high rpm and vroom sound from the engine. A winner every time. :poke:
 
It's not going to be the same as an old carborettor based engine where you'd just flood it by having the throttle open when overloading the engine. The computers are going to take over. With drive by wire, think of it in terms of FADEC (if still in use), the car's systems are going to try and keep everything optimal but it will depend on the loading you've put on the engine through your gear selection as jonnyfive said. Those with auto will have less to worry about as it will simply kick down to the most efficient gear as well as adjusting the fuel air mix.
 
Laws of physics say you can't create or lose energy in a system. So in a theoretical world, to get the car from the bottom of the hill to the top is going to take the same amount of energy regardless of which gear you are in assuming you travel at the same speed.

Of course we don't live in a perfect world. Engines are inefficient and heat energy loss from friction is the main issue. If the engine is revving faster it will incur greater friction losses and therefore waste fuel generating heat.

On the other hand, all engines have differing efficiency at different engine speeds and slowing the engine down only pays off so long as you don't get to a point where the engine is running so slowly that it cannot maintain its turning force without pumping in excessive amounts of fuel.

Generally peak torque is the most efficient point of an engine's operation. Most turning force for least revolutions. Go up the hill in the gear that best matches that engine speed and don't try to accelerate.

Get on a push bike and cycle up hill. You'll soon figure he answer out :)

Just my opinion!
 
I'm with GifBMW, essentially same amount of fuel, except for engine efficiency. If the engine is in it's normal rpm range, the difference is small compared to the energy needed to move a 1.5kkg car up a sizeable hill. Since the high gear rpm is slightly closer to max torque point, at least in my car, I'm slightly better off in high gear.

I wonder how close engine efficiency is to the torque curve? Is the efficiency of a 250 max torque engine at 200 torque 80%? My gut says it's related, but not linear.
 
Lol guys, are you waiting for a girl to give you the answer, she says sniggering :D
 
sars said:
Lol guys, are you waiting for a girl to give you the answer, she says sniggering :D
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Go on then - I love it when you talk dirty (engineering) :evil:

Most of the time I understand the answer too :P
 
You could always take a longer and faster run up.....
Find a level road around the hill..... :tumbleweed:

Feathering the throttle does lower mpg (obviously) but don't believe it will reduce with a labouring engine -

Where an engine makes torque and horsepower (at what revs) will have a bearing on mpg in such a situation.

I was asked to find a graph of my engine's revs/horsepower/torque to find out where it was being most efficient and most powerful as part of my IAMS training then to apply same in different driving styles and circumstances.
 
Guys

The answer is simple, you need to be in a gear that gives you good speed, so that the run is as short as practical, but does not load the engine excessively such that you maximise the engines torque, which is at the lower to mid rpm range.
 
For highest mechanical efficiency the piston speed needs to be kept low, at full load. For maximum economy piston speed should be 5-6m/s. So for piston stroke of 72mm (2.2 litre), 1000 rpm equates to 4.8 m/s, and for 3.0 litre, stroke 90mm, 6m/s. So keep the revs down for best economy.

However, at low revs the engine will develop little toque, and so may 'labour' under load, such as going up a hill, so you need to gear down to bring the revs up a little to produce more torque. Steeper hill, even more torque needed. The actual revs needed to give enough torque will depend on the engine.
 
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