underreving an engine - why is it bad?

rocky

I read in a few posts here and somewhere else that overreving is bad for the engine (and there are rev limiters for this) and so is underreving (the automatic gearbox will select a lower gear in this case, even if it's set to 'manual'). Now can you give more details on why underreving is bad? If I want to drive at 40mph in 5th gear, and the engine does not stall or shake, why is this bad?

I can see that when underreving you're not getting the max out of the engine, but other than this, are there technical reasons?

Thanks.

crosstownk

Under-revving can put excess strain on an engine causing it to pink and knock. Most cars have knock sensors which will adjust the timing in order to compensate. Essentially you are asking the engine to do work that it is unable to do for the gear it is in. This causes undue strain on components. But it's not as detrimental as overrevving - at least not in the short term.

maidhc

rocky wrote:

If I want to drive at 40mph in 5th gear, and the engine does not stall or shake, why is this bad?

If the car isn't labouring then 5th gear at 40mph is probably fine.

fletch

maidhc wrote:

If the car isn't labouring then 5th gear at 40mph is probably fine.

Yeh so long as you aren't trying to accelerate up a hill...

craichoe

fletch wrote:

Yeh so long as you aren't trying to accelerate up a hill...

Get a Turbo Diesel

heggie

or just drive properly?

unkel

maidhc wrote:

If the car isn't labouring then 5th gear at 40mph is probably fine.

I'll third that!

Have to say that I hate being a passenger in a car where the driver shifts up way too early (i.e. to 5th @30km/h). It makes acceleration sooooo slooooooww

Or where the driver doesn't shift down (in 3rd or even 4th around roundabouts) to the point of the car nearly labouring

From my experience I find many in the first group are taxi drivers, many in the second group are lady drivers :eek:

Max_Damage

unkel wrote:

I'll third that!

Have to say that I hate being a passenger in a car where the driver shifts up way too early (i.e. to 5th @30km/h). It makes acceleration sooooo slooooooww

Or where the driver doesn't shift down (in 3rd or even 4th around roundabouts) to the point of the car nearly labouring

From my experience I find many in the first group are taxi drivers, many in the second group are lady drivers :eek:

I think I'd fit into the first group category. No rush!

rocky

follow-up question - is the fuel consumption the same for a given rpm, no matter the gear?

I'm not saying I drive like this, I'm just wondering... can you improve fuel consumption by driving at 40mph in 5th, rather than at 40 mph in 4th?

zilog_jones

AFAIK no - I assume you get the best mileage around where the highest torque output is, or something like that. Driving at too high revs is inefficient, and of course the same would go for labouring the engine.

Those economy lights in '80s Toyotas were great - what happened to them? Then again most cars now have fancy trip computers that'll do mileage anyway.

slipss

Yes, you do get better mileage/fuel consumption in 5ft gear, or over drive as it's known. This is the reason you have an over drive gear on your car, thats the only purpose it serves.

peasant

The engine can only use as much fuel as it can burn. Properly set up it doesn't flush unburned fuel down the exhaust.

Hence you use less fuel at low revs and the advice is to shift into a higher gear as soon as possible, with the caveat of not labouring the engine.

In most petrol engines (with the exception of high revving sport ones) aroundabout 2000 rpm is where you can start driving it economically.

Diesels start pulling as low as 1200 - 1500 and usually reach their peak around 2500 ...that's why they are so fuel efficient.

E92

Yes most Beemers have an mpg guage which tells you which gives a constant readout of what you mpg is(or l/100km these days of course). As was mentioned most of their rivals have an mpg guage built in somewhere if they have a trip computer. @zilog_jones Toyota are bringing back something similar; the Auris/Corolla has a gearshift indicator as standard

Del2005

The only issue with under-revving a car is that carbon can build up in the exhaust system.

Not too sure on newer cars but on older cars, if cars only gets short runs and not pushed in any gear (but not maxed out) then they will get sluggish. And the only cure is to drive it hard, in 2nd or 3rd, till all the cr@p gets burned out.

BrianD3

follow-up question - is the fuel consumption the same for a given rpm, no matter the gear?

I'm not saying I drive like this, I'm just wondering... can you improve fuel consumption by driving at 40mph in 5th, rather than at 40 mph in 4th?

In 5th you may have to open the throttle more to mantain speed/revs if you come to an incline. This will kill economy and you'd be better off in 4th gear at higher revs but with less throttle input.

fletch

craichoe wrote:

Get a Turbo Diesel

I drive a TDI....tried driving up a hill in 4th @ about 50kph and flooring the throttle...yes it accelerated but the smell of burning clutch when I got to the top was not good!

peasant

BrianD3 wrote:

In 5th you may have to open the throttle more to mantain speed/revs if you come to an incline. This will kill economy and you'd be better off in 4th gear at higher revs but with less throttle input.

I'm not so sure that that is correct. (But I may be wrong also)
I've always thought, that it doesn't really matter how far you open the throttle ...if you're in low revs (and high gear) and only accelerating slowly, the engine will only burn as much fuel as it can use at that point/rev ...no matter how hard you step on the "loud" pedal. So, basically, even though a lot more fuel would be available (because of the wide open throttle) the engine mamagement only utilises part of it, as otherwise the engine would flood. Fuel consumption only rises slowly and gradually as the revs finally start to get higher.

In a lower gear, at higher revs on the other hand, there actually is the possiblity for rapid acceleration. So you open the throttle, more fuel gets used, revs go up, even more fuel is demanded, and so on.

BrianD3

peasant wrote:

I'm not so sure that that is correct. (But I may be wrong also)
I've always thought, that it doesn't really matter how far you open the throttle ...if you're in low revs (and high gear) and only accelerating slowly, the engine will only burn as much fuel as it can use at that point/rev ...no matter how hard you step on the "loud" pedal. So, basically, even though a lot more fuel would be available (because of the wide open throttle) the engine mamagement only utilises part of it, as otherwise the engine would flood. Fuel consumption only rises slowly and gradually as the revs finally start to get higher.

In a lower gear, at higher revs on the other hand, there actually is the possiblity for rapid acceleration. So you open the throttle, more fuel gets used, revs go up, even more fuel is demanded, and so on.

That makes sense and I have read that before. But from my own "experiments" I have found that a wide open throttle and low revs going uphill seems to result in worse economy than a lower gear, higher revs and less throttle. And that's in a modern fuel injected car.

DublinDilbert

I'll second that earlier post 'Brian 3'!

Most people out there think that just cause they are in 5th gear, and the engine revs are lower that they are using less petrol! They aren't it all depends on driving conditions as said above...

If I'm climbing a hill at about 55mph my car would be more efficent in 4th gear than 5th, now if I'm on the flat at 55mph my car would be more efficent in 5th... i think more cars should come with MPG gauges, might alter alot of peoples driving styles too, they'll see how much petrol they are wasting driving at 80 as apposed to 60 mph!

another way of putting it:

Power = Torque * engine_revs
( I know technically Power = Torque * Angular velocity, but just to keep it simple! )

If you require a certin power to climb a hill at a steady speed, if you shift to a lower gear you increase the engine revs, hence reduce the amount of torque required from the engine! Depending on where the engine is in it's torque/speed curve this may well be more efficent..

peasant

BrianD3 wrote:

That makes sense and I have read that before. But from my own "experiments" I have found that a wide open throttle and low revs going uphill seems to result in worse economy than a lower gear, higher revs and less throttle. And that's in a modern fuel injected car.

Low revs, high gear, uphill ...that would fall under "labouring the engine" I should think.

When going uphill, as soon as opening the throttle doesn't result in actual pickup of speed or revs, you definetly are in too high a gear.

It would still be more economical though, to shift from 5th into 4th or 3rd (depending on the hill), than to scream along in 2nd.

kraggy

Hey guys.

I have a standard 02 Opel Astra and was wondering what is the most economic way to drive it?

Would it be true to say between 2 and 3000 rpm or does it depend on what gear i'm in?

Not a clue about engines so apologies if question is too simple for ye!

thanks,
k.

enda1

I was under the impression that when a car is running at too low an rpm, that the oil is not under a high enough pressure to provide hydrodynamic lubrication thus resulting in a higher rate of engine wear. I think hydrodynamic lubrication occurs at around 2000rpm.