When to change gears for fuel economy?
02-11-2004, 05:29 PM
#16
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Sorry, Luis, my last post wasn't very clear.
I was trying to say that there are many more things to take into account when considering fuel economy than just shifting at the right rpms.
There is probably an optimal rpm for the na engine (but I don't know what it is) and your shift points will be based around this (taking into account load at speed caused by final ratios, rolling friction and aerodynamic drag).
However, I'm not too sure about that quote:
"If minimum consumption is to be achieved you must remember that, in a gasoline engine, the specific consumption decreases as the load on the engine is increased. This means that under given conditions and for a given speed of the car, the engine uses less gasoline when it runs comparatively slowly -say in top gear- with a wide throttle opening than when it runs at a smaller throttle opening and at higher revolutions in a lower gear."
Is the book saying that if you start of in 3rd, instead of going through 1st and 2nd, you will save fuel??? I know I'm over simplifying, but I can't understand how that can be. If you have wide throttle openings at low engine speed your engine is not being eifficient - it doesn't matter how you look at it.
I think that this topic is more complicated than you want the answer to be.
Most early fuel economy guages are based upon vacuum pressure, so only really show pressure diferences within the induction system. They're great as a rough guide, but that's about all.
If you want a definitive answer, I would suggest getting factory specs for the induction cycle to find out where in the operational rpms the engine is at it's most eifficient and work from there. There must be an answer, but using someone elses shift points ain't gonna save you fuel.......
I was trying to say that there are many more things to take into account when considering fuel economy than just shifting at the right rpms.
There is probably an optimal rpm for the na engine (but I don't know what it is) and your shift points will be based around this (taking into account load at speed caused by final ratios, rolling friction and aerodynamic drag).
However, I'm not too sure about that quote:
"If minimum consumption is to be achieved you must remember that, in a gasoline engine, the specific consumption decreases as the load on the engine is increased. This means that under given conditions and for a given speed of the car, the engine uses less gasoline when it runs comparatively slowly -say in top gear- with a wide throttle opening than when it runs at a smaller throttle opening and at higher revolutions in a lower gear."
Is the book saying that if you start of in 3rd, instead of going through 1st and 2nd, you will save fuel??? I know I'm over simplifying, but I can't understand how that can be. If you have wide throttle openings at low engine speed your engine is not being eifficient - it doesn't matter how you look at it.
I think that this topic is more complicated than you want the answer to be.
Most early fuel economy guages are based upon vacuum pressure, so only really show pressure diferences within the induction system. They're great as a rough guide, but that's about all.
If you want a definitive answer, I would suggest getting factory specs for the induction cycle to find out where in the operational rpms the engine is at it's most eifficient and work from there. There must be an answer, but using someone elses shift points ain't gonna save you fuel.......
02-11-2004, 06:06 PM
#17
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im not going to be helping, but in my Audi i cruise at aroung 1500 RPM's. that is where my manual says is the best gas mileage. i can usually get about 26 MPG. people ask me why i am in fifth and going 37 MPH.
02-11-2004, 06:36 PM
#18
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this is PART of what i remember from a discussion on this in a highschool shop class a couple years ago (i wonder if its not quiet right because it was a class on carbs not fuel injection> the basic principle is the same)
please excuse me if im wrong:
ex: uphill, (your engine is loaded) you need to be in a gear low enough (high enough RPMs) to have enough power to get up the hill, otherwise your engine will literally suck in fuel trying to gain the rpms to maintain the speed up the hill
down hill: put your car in as high of a gear as you want, the car doesnt need to make any power right now. this is why i believe the factory uses vacuum as a fuel consumption gauge (go out and absolutely floor it through the gear of your choice and then quickly let off the gas: during acceleration the engine is loaded and sucking in fuel and air..when you let off there is absolutely no load the engine simply returns to rest and its likely that you will slow down faster then your engine and its fuel needs will be near nothing..your engine will go from needing tons of vacuum: sucking in air to needing none.)
whoever said listen to the engine: i believe that point is VERY good
you dont have to be going up or down a hill...if whatever load you have is too much for your engine its going to be sucking in fuel to compensate
another thing to remember is that your grandma prolly gets great gas mileage for awhile but the carbon build up in the engine is VERY hard on it
please excuse me if im wrong:
ex: uphill, (your engine is loaded) you need to be in a gear low enough (high enough RPMs) to have enough power to get up the hill, otherwise your engine will literally suck in fuel trying to gain the rpms to maintain the speed up the hill
down hill: put your car in as high of a gear as you want, the car doesnt need to make any power right now. this is why i believe the factory uses vacuum as a fuel consumption gauge (go out and absolutely floor it through the gear of your choice and then quickly let off the gas: during acceleration the engine is loaded and sucking in fuel and air..when you let off there is absolutely no load the engine simply returns to rest and its likely that you will slow down faster then your engine and its fuel needs will be near nothing..your engine will go from needing tons of vacuum: sucking in air to needing none.)
whoever said listen to the engine: i believe that point is VERY good
you dont have to be going up or down a hill...if whatever load you have is too much for your engine its going to be sucking in fuel to compensate
another thing to remember is that your grandma prolly gets great gas mileage for awhile but the carbon build up in the engine is VERY hard on it
02-11-2004, 10:49 PM
#19
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Higher loading results in higher effective compression, and that is what will increase the thermal efficiency of the engine. Shift points do make a difference, but their effects are much less than what rpm and loading will apply when cruising steady state. In general, shifting at lower rpm will increase fuel economy, but one must keep in mind mechanical limitations such as vibration. I know my turbo engine does not like lots of throttle below 1800 rpm so shifting at about 3000 would keep it happy.
All of the fuel economy engines for competition run "burn and coast" where they will run full throttle against a very tall gear, then coast with the engine off, and repeat this cycle within a specific speed range. The key is to minimize the energy to accelerate and to use most of it for overcoming drag and friction.
As for the carburetor sucking in fuel at low rpm, they suck even more fuel on the overrun, whereas fuel injected engines usually shut off the fuel delivery when coasting. Carburetors also don't have good full-throttle enrichment, especially at low rpm since they are dependent on vacuum to draw the fuel out. In cases of larger carburetors, they will usually run lean at low rpm/large throttle angles, which is why they will often cause the engine to buck.
As Mossy stated earlier, it's smoothness that will increase your economy, and that includes using as little brake as possible. We all know where that energy goes...
All of the fuel economy engines for competition run "burn and coast" where they will run full throttle against a very tall gear, then coast with the engine off, and repeat this cycle within a specific speed range. The key is to minimize the energy to accelerate and to use most of it for overcoming drag and friction.
As for the carburetor sucking in fuel at low rpm, they suck even more fuel on the overrun, whereas fuel injected engines usually shut off the fuel delivery when coasting. Carburetors also don't have good full-throttle enrichment, especially at low rpm since they are dependent on vacuum to draw the fuel out. In cases of larger carburetors, they will usually run lean at low rpm/large throttle angles, which is why they will often cause the engine to buck.
As Mossy stated earlier, it's smoothness that will increase your economy, and that includes using as little brake as possible. We all know where that energy goes...
