Gearing and Engine Wear

[astral83]

Hi

Was wondering if anyone could help explain a few things. I have heard it is often better to drive in a lower gear and keep the revs up rather than changing up too early because

1) It gives you more control of the car
2) It is better for the engine - higher gears at low speeds cause stress on the engine

Is this true and why does a higher gear cause more stress on the engine. I would have thought more stress would be caused by driving at higher revs. Can anyone explain?

Thanks

[Jungles]

Every time fuel burns in the engine, it releases energy that must manifest itself somehow. If you are driving in a high gear but forcing the engine to labour at very low RPM (eg. < 1500rpm @ 4th gear), the energy produced is not being efficiently transferred to maintaining or increasing your speed. Instead, the engine itself and various mechanical components in the drive train is absorbing (for want of a better word) this, causing excess wear and tear. Driving at unnecessarily high RPM also causes excess wear and tear by forcing the engine to work closer to its upper limits of mechanical integrity, especially if the engine has not been run-in to work at high RPM.

Most petrol-powered road cars have engines that are designed to operate most smoothly between 1500 to 4500 RPM, depending on gearing. You'll find that at lower gears, lower RPM produces a smoother drive, while at higher gears, higher RPMs are smoother. For example in my Toyota, 1st gear is smoothest at around 1500-2500 RPM, while 5th gear responds best at > 3000 RPM.

So the advice you hear about gear selection is not about which is better - lower gear and high RPM, versus higher gear and low RPM. It's making your engine run comfortably, both mechanically as well as for you, the driver.

Generally, drivers who labour their engine at higher gears and low RPM not only produce excessive wear and tear on the engine, but also sacrifice control over their speed and acceleration, which can be important to have when faced with an emergency scenario. However, running at lower gear and high RPM can cause drivers to change their speed in an un-smooth manner, causing the car to become unsettled and losing traction.

It's up to the driver to use their vehicle's documentation, and their own experience, to find the ideal gearing and RPM range when driving.

Edited by Jungles on Sunday 4th June 07:45

[Big Fat F'er]

Jungles said:

...some excellent stuff that was spot on...

Plus, most 'normal' drivers in 'normal' cars tootle along in a 30mph zone in 4th at 1500rpm. It feels okay, 'cos they don't know better. Try it in 3rd, which will probably give somewhere around 2000rpm. Suddenly it's easy to hold the limit, you can use acceleration sense to slow down, you can smoothly move through hazards, etc., etc., etc. It's even more economical on gas.

It takes a while to get used to (depending on how long you've been driving) and you'll have to work on a smooth right foot but I guarantee that if you stick to it until you get used to it, you will realise the benefits. If you do it, then try to go back to doing it in 4th, it will feel really really wrong. There won't be any power, you'll use the brakes more, and the speed will drift more.

See how it goes. It's always good to try new stuff.

[GreenV8S]

In general, the more power you need available, the more revs you should use.

The lower the revs the less energy is expended moving pistons up and down the bores so the better the potential economy. It also reduces wear and tear on the engine AS LONG AS you don't then start putting a significant load on the engine. This is fine for wafting along on a long journey with no significant load on the engine but not for stop/start driving around town.

Working the engine hard at low revs is very bad for it. It encourages bore wear and basically wears the engine out. If you need more than a trickle of power from the engine, it it far better for the engine to use moderate torque at moderate revs rather than high torque at low revs. Automatic gearboxes are excellent from this point of view because they automatically change down to prevent you from laboring the engine. Pretend you are an automatic gearbox and you won't go far wrong.

[gridgway]

Big Fat F'er said:

Plus, most 'normal' drivers in 'normal' cars tootle along in a 30mph zone in 4th at 1500rpm. It feels okay, 'cos they don't know better.

One of the things that is going on here is the differnce in tootling speeds in a 30 limit. Most people tootle somewhere in the higher 30's (35+), I have found in many of the cars I have driven this actually works ok in 4th. If your tootling is a at a max of 30 in a 30, it's a whole nother thing!

Just a thought, there are numerous statements about engine wear and labouring, anyone know the mechanics of what's going on here? What actually is engine labouring, why does it happen and what are the mechanisms that cause the extra wear on the engine?

Graham

[GreenV8S]

gridgway said:

Just a thought, there are numerous statements about engine wear and labouring, anyone know the mechanics of what's going on here? What actually is engine labouring, why does it happen and what are the mechanisms that cause the extra wear on the engine?

I'm not sure what the exact mechanism is, but I think it's related to the issues that occur when you're bedding new rings in.

Because the con rods are angled sideways during the stroke, the vertical loads on the pistons result in a side load on the piston which pushes it sideways against the bore. The vertical load comes from the inertial loads on the piston, and the torque transmitted to the crank. The more torque, more more side load.

Similarly, the rings are designed to seal by expanding slightly under gas pressure. The more torque the engine is producing the more gas pressure in the cylinder, the greater the bursting force on the ring and the more wear occurs.

The final effect is that the lubrication properties of the oil film inside the cylinder will vary with speed.

When you are bedding in a new set of rings, the usual advice is to put load on the engine briefly but often, but avoid laboring it. The point here is to make sure that the bores wear themselves in to the piston rings and not to the piston. If you never load the engine the rings don't push on the bore so the bore beds itself in to the piston instead. If you labour the engine the side loads wear the honing off the bores before the rings have a chance to bed in.

I don't know for sure, but I imagine that the same factors apply during the whole life of the engine - bedding in doesn't stop after the first thousand miles, it is just a gradual wearing process that occurs very quickly at first and then tapers off as the moving surfaces bed in to each other, but never completely stops.

[gdaybruce]

I'm not a mechanical engineer so this is what might be termed intuitive guesswork!

At low engine speed and wide throttle opening the pistons will draw a large quantity of fuel/air into the cylinders on the piston down strokes. This large fuel charge is then compressed on the following up stroke and ignited by the spark plugs. At this point, because the engine speed is low, the burn is happening while the pistons are at the top of their strokes and before they have had chance to decend any distance. The result is very high pressures and accelerative forces on the piston/conrod/crank assembly, leading to higher rates of wear.

Conversely, when the engine is running faster, the pistons are already descending rapidly as the fuel burns, leading to smoother transfer of energy from the burning fuel to the pistons and less stress on all the moving parts connected to it.

Of course, this could be complete rubbish so I'll be interested to hear from someone who really knows!!

[dilbert]

Choice of gear is very difficult to prescribe.
In general, if you have any mechanical sympathy, I'd say that if the car "feels" like it's labouring then it's also wearing at a high rate. The remedy, choose a better gear.

[Big Fat F'er]

gridgway said:

Big Fat F'er said: Plus, most 'normal' drivers in 'normal' cars tootle along in a 30mph zone in 4th at 1500rpm. It feels okay, 'cos they don't know better. One of the things that is going on here is the differnce in tootling speeds in a 30 limit. Most people tootle somewhere in the higher 30's (35+), I have found in many of the cars I have driven this actually works ok in 4th. If your tootling is a at a max of 30 in a 30, it's a whole nother thing! Graham

Correct. The thing is, IF you want to drive within the limit in a 30mph zone, then 3rd at 2k revs makes it easy, 4th at 1.5k revs makes it difficult. The IAM did a study, looking at those drivers that broke the limit, but only by a few miles per hour. Nearly 3/4 thought that the correct gear to be in was 4th. They then had to keep the speed up, to stop it labouring. I'm not hijacking the thread to make it a limit one, just making the point.

Not only that, but a labouring engine is burning fuel inefficiently, and there needs to be a bigger throttle opening. So although folk think it's more economical, you actually use more gas labouring in 4th than running sweetly in 3rd but with higher revs.

Bit of an over simplification, but true nevertheless.

[7db]

I sometimes potter through 30 limits in 4/5/6, dropping the engine revs, making more discrete impact on the villages - esp if early or late. You need to anticipate where you are likely to need acceleration and act accordingly, but if a potter through, I see no harm in using a higher gear and letting the engine calm down from the NSL thrashing you've just given it.

Of course on exit from the village, you have a very long accelerative zone without chaging gea rwhich give you more time to assess first corner, roll a joint etc.

[turbobloke]

In a high gear at too-low road speeds you're relying on low-down torque to keep the car moving as well as overcome internal resistances of many types. Most family type cars have relatively little torque at peak and much less down nearer tickover, and all the knocking and groaning you hear represents the components of the drivetrain fighting a losing battle due to being given a poor mechanical advantage to work with. Good job PHers have sights on other things.



If for the sake of discussion you drive a car with high-ish torque, say 200 lb ft at tickover rising to well over 400 lb ft at peak, the car doesn't mind what gear it's in, will roll away from standstill using only tickover revs without labouring, and do so in 1st or 2nd gear (not that one would do this 'normally'). Such a car is also quite difficult to stall in the first four - and quite a pleasure to drive. Sure beats the whiny high revving alternative, where you end up rowing the car down the road with the gear stick due to lack of any alternative. All imho of course.

[astral83]

Thanks for answering my question. I tend to drive in 3rd gear in a 30mph zone anyway - find it keeps me in the speed limit and gives me better control should i need to accelerate/deaccelerate. Probably why you get so many old people flying through a 30 zone at 40mph then sitting there at the NSL - probably saves me ever changing from fourth

To be honest however I never knew a too higher gear could actually be worse for the engine (labouring) than a lower slightly higher revving gear.

[turbobloke]

astral83 said:

Thanks for answering my question. I tend to drive in 3rd gear in a 30mph zone anyway - find it keeps me in the speed limit and gives me better control should i need to accelerate/deaccelerate. Probably why you get so many old people flying through a 30 zone at 40mph then sitting there at the NSL - probably saves me ever changing from fourth To be honest however I never knew a too higher gear could actually be worse for the engine (labouring) than a lower slightly higher revving gear.

This isn't an advert - but with a half-decent engine to begin with and a decent synthetic oil, you could run a car (normally, not labouring all the time!) for a very, very long time without significant engine wear...didn't Mobil 1 do that test with a million mile BMW - on a rolling road type set-up - where after the million miles all components were within manufacturer's tolerances?