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?

  • 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
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    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
up vote 2 down vote accepted
+50

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.

  • 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
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    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.

  • 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

It is more efficient, per se...

B-29 crews in World War 2 found that when unloaded, their four-engine aircraft could fly farther on three running engines and one engine stopped (and its prop feathered to minimize drag) than they could with all four turning. Why? Because cruising speed with just three engines required opening their throttles more, reducing pumping loss.

Engineering schools run well-publicized fuel economy contests that are now won by fabulous numbers such as 1200 mpg. This is achieved by very lightweight, low-drag shapes that are accelerated by tiny engines running mid- to full throttle. Then the engine is disconnected and stopped while the vehicle coasts down to some chosen low speed, at which point the tiny engine is again started and accelerates the vehicle again. The engine’s small size, need to run at large throttle opening, and intermittent operation minimize both pumping loss and mechanical friction. Mileage fanatics, driving instrumented Honda Insight two-seater cars in this same “pulse-and-glide” style, have been able to get close to 100mpg on long trips.

Of course, pumping loss isn’t the whole story in the above cases. Cylinder deactivation also saves some piston ring and bearing loss in the deactivated cylinders, and clearly, three B-29 engines have less total mechanical friction than four.

&

Charles Lindberg used a similar technique to significantly increase the range of the P-38 fighters in the Pacific. He showed the pilots that reducing rpm, leaning the mixture and INCREASING the manifold pressure drastically reduced fuel consumption.

I suspect that this is not implemented in consumer cars because it is not very "easy" to use and is not as economical for manufacturers to provide.

As you can see from the above examples, to really be efficient this method favors having multiple smaller engines all running at mid to high throttle over having one single powerful engine running at a range of throttle levels. The costs for manufacturers to provide a vehicle with multiple engines to efficiently account for each speed range would be enormous and is probably not worth the savings imparted to the customers through fuel savings. Not to mention the increase in maintenance by a factor of at least the number of engines in the vehicle.

But what if we said screw the multiple engines, lets just try to make the one engine more fuel efficient by modifying the gearing?

Then the car, at any given speed, will have one optimal gear for acceleration and another optimal gear for cruising for fuel efficiency.
I think on average this method might gain the average person about 1 mpg, maybe 2, at the cost of real-time throttle responsiveness. I think if you try to balance the scale between throttle response vs 1-2 mpg, the benefits of the mpg over millions of people is definitely there (1-2 million miles or roughly 50k-100k gallons of gas or roughly $200k-$400k per million people), but is it really significant enough to outweigh the few accidents that may have been prevented by having better throttle response (and the added "comfort" of having some throttle cushion during normal driving)? It would really depend on the statistics and analysis therein, but I would guess probably not.

Additionally, especially for automatics, this method would have to be heavily controlled by the computer to be acceptable for use by everyday consumers and would cost that much more money in R&D, testing, and production as another feature they provide. If a feature is provided but does not work correctly or smoothly enough then it damages the brand and might cause direct damage through warranty work or recalls.

Honestly, I think you have a better handle on pumping loss and frictional loss in gasoline IC engines than the other responders. Might have had better responses in Engineering SE.

Sorry for the necromancing here, but i stumbled upon this thread and i think there's another important reason why this shouldn't be done, and why it probably isn't even tried out by manufacturers. The point is that the situation you're describing constantly keeps the engine at it's maximum torque/load. It's highly probable that it's more efficient there(if the AFR would be managed for that) so i understand your idea, but there's one big downside.

Torque is just a slightly different variation of force. Imagine what will happen if all the parts in the engine constantly have to sustain the forces that come into play when the engine operates at maximum torque. They can't endure that stress for extended periods of time. Things will wear out rather quick.

Shift in top gear at 70km/h or slower so the engine revs at 1000rpm, and put the pedal to the metal. The engine will growl in protest without the car accelerating. It doesn't sound healthy, and it isn't. At 2000rpm the engine has to deliver only half of the torque, and 2000rpm isn't even much. The engine will sustain that situation for much much longer, but yes it's probably a bit less efficient.

Diesel generators manage their engine like you proposed. They operate at a constant rpm and maximum torque, because that's where they achieve maximum efficiency. But they're designed for that, with that situation in mind. A car is not, as it isn't needed. It would make the engine a lot heavier at the cost of material, fuel consumption, road tax, etc.

  • The situation described by OP has nothing to do with keeping the engine in max torque range or not. It is all about reducing pumping losses and frictional losses of an gasoline IC engine – agent provocateur Oct 10 at 23:17
  • @agentprovocateur I didn't say anything about max torque range, and i'm acknowledging the issue of pumping and friction losses. So i don't get your coment. I'm saying that lugging the engine, which is what OP is suggesting, isn't good for it. Yes pumping losses and some friction loss may be reduced, but there are strong downsides. It wouldn't even improve things in modern cars, as they have variable valve timing. They throttle the air with the valves, with minimal pumping losses. What exactly is it you're disagreeing with? – Bart Oct 12 at 10:07
  • You literally directly mentioned max torque in your first paragraph as what the OP is describing: "The point is that the situation you're describing constantly keeps the engine at it's maximum torque/load. It's highly probable that it's more efficient there(if the AFR would be managed for that) so i understand your idea, but there's one big downside." The situation the OP is describing does not keep the engine at max torque. Your entire response is built upon this premise. – agent provocateur Oct 15 at 22:44
  • @agentprovocateur That's because that's exactly what OP is describing:"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." At WOT, you're at max torque at any given RPM, (you can't keep an engine at 'max torque range' like you mention. Learn the difference.) That's what my answer is about, and I can defend it like I did. You can't, don't start objections if you can't defend them or understand the situation. So i'm sorry, but you're wrong. Case closed. – Bart Oct 22 at 7:34
  • why can't I defend them? why do you determine that I cannot? what is it that you know about me that allows you to make these judgements? this is a discussion about facts and you have not really given any, just statements that you think are facts. – agent provocateur Nov 2 at 22:59

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