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I'm specifically referring to the loss due to the pistons pulling air through a mostly closed throttle while cruising at highway speeds in a gasoline engine.

To give an example, I've always wondered why this wouldn't be more economical fuel-wise:

Add another gear to the gearbox, say a sixth gear to a 5-speed car. This gear would be useless for accelerating whatsoever; it would be such a high ratio that at 65 MPH, wide-open throttle, it would hold the vehicle at a steady pace or only slightly accelerate. This would reduce much of the pumping losses from making the pistons suck through a nearly closed throttle, and would seemingly allow for better fuel economy, due in part from both 1) lower RPM so less friction loss and 2) less pumping loss.

I am aware other things may need to accompany this; for example WOT in 6th gear should not richen the mixture as WOT does normally, etc., so let's say the car was produced from the factory with the computer and associated electronics accommodating the fuel-gear.

However, this has never been implemented. Is it just not efficient? If not, why not? Or would it be an irritation to consumers that expect a more responsive car, requiring a shift to accelerate? Too expensive to add the gear, or just never been tried?

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A gear that is "useless for accelerating" at highway speeds would sound very dangerous to me. You always want a bit of reserve to be able to accelerate past/through hazards, and I would think that the safety factor would outweigh a marginal gain in fuel efficiency for an urban or suburban driver's typical trip. –  choster Jun 7 '12 at 13:49
An an automatic, the vehicle could automatically downshift; for a manual, the driver would just have to know to downshift to accelerate. Perhaps what you say is true; the gain is so minor, it's not worth the added hassle. It'd be nice if there was a way to measure it. –  Ehryk Jun 7 '12 at 17:00
choster: Lot's of [automatics] have exactly this, though. They have an over-drive gear. Usually a final drive of less than 1:1. - Pretty pathetic acceleration, which is why they also have a kickdown switch. You step on gas, it's smart enough to downshift. Not exactly what author is calling for, but same thing could be applied in principal. (A kickdown for the "fuel gear.") –  Robbie Jun 8 '12 at 19:01
You've nailed it; what I'm asking is for another gear, one further. Perhaps an over-overdrive or fuel gear at 0.5:1 or whatever it would be based on how much power is required to maintain cruising speed at full throttle. When I look at 6 or 7 speed vehicles, they're usually just narrower gears that cover the same range of a 5 speed. –  Ehryk Jun 8 '12 at 21:16

2 Answers 2

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Some solutions have been implemented - input airflow is reduced in many engines (especially turbo versions) so you aren't actually having to send a lot of air through the engine.

In any case the 'suck' when the throttle is only slightly open is minimal, so you aren't losing a lot of energy with this.

You would waste more energy having to use a wide open throttle for cruising - instead many cars have an overdrive. In my car, 6th gear at motorway speeds runs at about 2,200rpm which is hardly doing any work.

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But your 6th gear is probably quite useful, especially in turbocharged cars. I'm asking - why not go FARTHER, an over-overdrive, where your RPMs are down to 1,500 or so and at WOT you just barely accelerate (or stay even) at 65MPH –  Ehryk Jun 7 '12 at 9:13
But why bother with WOT? The key point of WOT is you are forcing more fuel into the cylinders, so you'd be better off not doing that. Or am I misunderstanding you? –  Rory Alsop Jun 7 '12 at 10:54
No, that'd only be on a diesel car. The point of WOT is that you no longer restrict the airflow into the cylinders, and the computer will figure out how much fuel to inject based on the airflow readings. –  Ehryk Jun 7 '12 at 17:01

I can think of a couple reasons (mostly apply to automatics though)...

If you go WOT to maintain speed, there's nothing left for passing/going uphill/to tell an auto to downshift. Also, the car would be in open loop mode and waste fuel.

To keep the car in closed loop, you're going to max out at around 75% throttle. Any time you did pass or have to climb a hill, you'd have to downshift and/or go WOT too.

A "social" reason would be that people like to have the feeling of there being a lot of pedal left/"reserve power". Having the gas pedal down when just doing normal highway speeds is not what the normal/uninformed consumer wants to be doing.

Pumping losses do seem to be measureable, at least on my car. If I mash the gas to 50-75% starting from stop lights, but still shift at the same low RPMs as if I was accelerating slowly I can consistently show a gain of 2 MPG on my daily drive over my normal driving style.

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I get the reasons people don't want to mash the pedal to the floor, but this could be implemented internally by any drive-by-wire car; the driver doesn't even have to notice or 'floor it'. Closed loop mode would have to be extended to 100% throttle for this gear. –  Ehryk Jun 7 '12 at 16:58

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