Gas Consumption Theory Question

Discussion in 'The Garage' started by Can Can, Nov 15, 2002.

1. Can CanPusher ManStaff MemberSuper Moderator

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Today while I was roading the backhoe around town I had a thought-

Does an engine always use the same amount of fuel at a certain RPM? If you rev your motor up to 2000 RPMs at idle, will the motor go through the same amount of fuel as it will cruising down the highway at 2000 RPMs?

I know that in idle the motor doesn't have to transfer power through the drivetrain, but at speed the truck has momentum which *MAY* help with fuel consumption. So is there an advantage either way? I also know that you have to take load and throttle position into consideration, so I'm just talking about idle vs. cruising speed.

Anyhow, I hope my question makes sense. I know there are many variables that could affect the actual outcome, but in theory, does anyone know if there's a difference? To me, 2000 RPMs should use the same amount of fuel either way.

2. hammer1/2 ton status

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aerodynamics! The faster you go the more fuel you will consume.

3. Can CanPusher ManStaff MemberSuper Moderator

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Are you sure? If you have wind resistance it also means you have momentum on your side. Let's assume that you have the exact same amount of pedal into it at 2000 RPMs in both idle AND cruising- shouldn't the engine use similar amounts of fuel?

4. fortcollinsram1/2 ton status

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Yes if you are pushing the pedal the same amount crusing as you are at idle, the engine will use the same amount of fuel, but if you think about it, if you aren't giving it gas, you wouldn't be crusing very fast would you..While engine rpm does affect the amount of fuel used, engine load plays a bigger part..If you are towing 5,000# up a mountain pass you WILL NOT get the same gas mileage as you would crusing on flat with no load, even if the RPMs were kept within a similar window...Have you noticed that crusing downhill on the interstate, you don't have to lay into the pedal much? Then as soon as you go uphill, you gotta give it a bit more? That is b/c the engine is having to move more load and thus requiring more fuel and air to keep that load at a constant speed...

Chris

5. hammer1/2 ton status

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thats what I ment but I didn't fell like typing all that /forums/images/graemlins/thumb.gif

6. Muley1/2 ton status

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It think what you are looking for is if at 2000 RPM's will you burn more stationary versus crusing down the road.

My opinion is that you would burn LESS fuel on the road. Here is my reasoning.

Get out on the open, flat interstate and get up to speed. Get the RPM's up to lets say the 2000 mark. Set the cruise control. Now watch/ feel the gas pedal. It will move. It will depress when it needs a little more fuel then back off when it doesn't. So based on that, I think it would burn less.

Now here is the possible catch, would it really be enough difference that you would be able to tell? Probably not.

Just my simple opinion.

7. Can CanPusher ManStaff MemberSuper Moderator

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Did I not say this? :

</font><blockquote><font class="small">In reply to:</font><hr />
I also know that you have to take load and throttle position into consideration, so I'm just talking about idle vs. cruising speed.

[/ QUOTE ]

Ummmmmmm.....did you read my whole post? It doesn't sound like it, brother. /forums/images/graemlins/rolleyes.gif

8. Can CanPusher ManStaff MemberSuper Moderator

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Yeah, that's along the same lines as I was thinking. Your Cruise Control example was a smart way to illustrate the concept. Good job, brother!!!! /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif

9. hammer1/2 ton status

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sationary you don't have to turn the drivetrain on the highway you do and areodynamics plays a big deal on your fuel economy. the faster you drive the more gas you use even when you go the same distance, there is more strain on the engine cuz there is more pressure on the veicle from the wind.

10. MEPR1/2 ton status

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you will always burn more fuel running. Think about it your moving a 5-6000lbs baheamoth of a truck that has a drag coeficiant of 100% (probly not that much but close!). Now if you we in my truck (all whoping 2600lbs of it) there might be a debate /forums/images/graemlins/grin.gif

11. TWISTEDJACK1/2 ton statusGMOTM Winner

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Try this take your rig up to 2000 rpm cruising down the road, shift into neutral without removing your foot from the pedal, your engine is now doing 4000-5000rpm!!! Now that should tell you that it would consume more fuel moving than sitting still.

12. 4x4Freak1/2 ton status

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When under load it takes more fuel to get/keep the rpms up. But with no load it takes much less.

13. Can CanPusher ManStaff MemberSuper Moderator

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I tried doing that in OD at 55MPH and the RPMs were only slightly higher.

You do have point, though.....

14. TXsizeK51/2 ton status

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Well, with manual its easier... in 2nd gear i can be at 2.5k and ill put in the clutch and keep my right foot wher eit is and it jumps to maybe 4 or 5k. OD wont be as much beucase you are alreayd at low RPMS not giving it much gas while going down the highway... manual works best for this test.

15. Blazer19701/2 ton status

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Any engine will use A LOT more fuel at a given RPM under load than it will with no load. The more torque the engine is putting out, the more fuel it will use. Have you ever looked at how the governor works on your lawn mower? This is a good mechanism to illustrate the concept. It is designed to work at a constant RPM, but will automatically open the throttle wider as the load increases. Once the throttle is wide open, if the load increases more, the RPMs will drop.

16. HarryH31 ton statusAuthor

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You need to think of fuel consuption in terms of the amount of work being performed. More work requires more engery. In this case, the energy comes from consuming fuel. Ever notice that your rig burns more fuel when towing a heavy trailer than it does when running unladen? That's because the engine has to work harder to maintain RPM at a given speed when dragging the extra load behind the truck.

17. Grim-Reaper3/4 ton statusAuthor

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I see what your asking and there is some things going on in the auto industry about this.
Engines do have a range where they are more fuel efficient. Problem is gearing. Honda in particular is playing with a variable gear ratio transmission that uses a funky belt (steel) so they can vary the diameter of the the pullys and maintain the same RPM on the engine but it will still accelerate due to a variable ratio transmission. The problem is making a strong enough transmission to do this. They are playing with High fuel efficent light cars with small engines and they are still having an issue with reliability.
The sweet spot on a 350 is around 2200RPM. Thats pretty well in line with their peak torque vs fuel consumption. If this ever comes to the market place what will happen when you press the accelerator to go faster is the engine will come up into it's peak torqure range and stay there and then the transmission will change it's ratio to most accomidate the peddle input. Then once you acchieve the speed you want and back off the peddl the engine will drop Rpm to a more fuel effiecent range that it is capable of of maintaining the speed. When you com to a hill the engine RPM and transmission ratio will change to maintain the speed even though you haven't changed the possition of the peddle.
They will take some getting use to that's for sure if you think about hearing the engine RPM's changing even though you haven't changed input.

18. m j1/2 ton status

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Subaru actually sold tons of those trannys.
did Honda's ever make it to market?

19. Grim-Reaper3/4 ton statusAuthor

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Ohhh yeah the Justy had one didn't it. I rode in one of those. Weird little car. Floor it and the engie would come up in RPMS and hold the same note but car still accelerated.

20. BlueBlazer1/2 ton status

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Are we talking about your truck with the 6.2?