Is "engine braking" bad for my car?

Factual Questions
21

If you downshift properly, engine braking causes almost no wear on the clutch. The problem is, 95% of the people who think they can drive stick can’t downshift properly.

By rev-matching—“blipping” the throttle to increase engine speed to match the lower gear—there’s no (or minimal) slipping on the clutch. What most people do when they downshift is let the transmission speed the engine up, causing a noticable lurch as the car drops a few sudden mph. The problem is that this also involves a lot of slipping of the clutch disc, and that’s where the wear comes from.

Double clutching is another technique that works to save wear on the tranny’s syncros, but that’s a minor consideration for a modern gearbox. If you can’t expect people to master something as simple as rev matching on a downshift, double clutching is entirely too much to ask.

Here’s an old thread that gets into a good discussion of both double-clutching and rev-matching.

22

Agree with everything you write, but I’ll add that the same applies to upshifting.

You can burn through a clutch in a few thousand miles, if you really try. You can make a clutch last 250,000 miles, too, even including engine braking.

23

Well, in the thread I linked to, I had posted that rev matching during upshifting occurs somewhat naturally, and requires less conscious efort. But it’s still important.

I also mentioned in that post that I had a car which had its original clutch replaced after 180,000 miles. I only put less than half of those miles on myself, so I don’t know how the car was driven before I had it. It’s pretty easy to use a clutch and cause virtually no wear, because slippage is almost always unnecessary—except for moving from a standstill. And even then, slippage can be minimized, compared to the average driver.

24

Can’t find a cite now, but I seem to recall from my Mechanics of Machines days at university, that a Diesel engine provides NO ENGINE BRAKING. So even though one has this 21:1 compression ratio, there is no engine braking available. That is unless the fuel metering valve is actually shut down, i.e no or very little fuel is injected into the cylinder. This also relates to the problem of overfueling in Diesel engines. Can somebody deny/confirm this?

Anyhoo, South Africans prefer “stick shifts” as you Yanks call them. One is considered a ninny or wet rag if you drive an auto. My Ford Fairmont is automatic, but I still leave some manual boy racers at the lights! Plus, heavy traffic is a breeze.

My Land Rover Discovery is manual. I have been taught to, and always downshift and use the engine to brake somewhat. Note that this does not mean letting the engine do all the braking and letting it rev up to, say, 5000rpm to try and decelerate the car, rather more like taking up some of the braking effort from the brakes and ensuring that I have absolute control over my vehicle at any speed at any time, e.g. ensuring that I am in the right gear when entering the corner to accelerate out of the corner, etc. And, of course during low speed steep descents, being a 4x4 you actually want all that braking power at your disposal

My 2c worth. In Rand terms that would be 14c, actually…

Dries Venter

25

Oops, what I meant was that the power loss is in the torque convertor and a manual transmission doesn’t include one :slight_smile: I was trying to pick up the workings of an automatic gearbox off howstuffworks.com but its a bit of a brain ache, the converter seems simple enough in principle tho’

26

More 2c worth, referring to audilover’s link to double de-clutching. The guy who taught me the ins and outs of 4x4 driving one day tied my left foot (clutch) back so I could not reach the clutch. This was in an old Land Rover 110. He then said, your clutcjh is now FUBAR, drive without it. After much gnashing of teeth (mine and the transmission’s) I got it 100% right, notwithstanding that I cut my driving teeth on an old 25 ton Mercede-Benz truck that still had a real “crash-box” with no synchro anywhere in sight! Man, that old 'Benz truck had a sweet transmission, if one took the pains to learn it properly. I had it down so fine, that I could sometimes skip a gear going up or dow, 1st to 3rd to 5th, or 6th to 4th to 2nd.

Those were the days…

27

Mr. Venter,

I assure you, I was able to make my passengers highly uncomfortable using my Mercedes. If I manually adjusted the idle speed controller (located next to the speedo) to the lowest setting, the vehicle provided ABUNDANT engine braking.
Bear in mind, this '78 had mechanical fuel injectors. A state-of-the-art electronically-controlled direct-rail (is that what they’re called nowadays?) injection diesel engine may be different from my old Benz in this regard, and that may more closely resemble the vehicles you learned about in school.
Alternatively, Mercedes is just a bunch of oddball Germans who insist on making strange, one-of-a-kind vehicles. Wouldn’t surprise me in the least.
God, I miss driving a Benz… must wait 'till my next big raise to get another.

28

(for the record, I absolutely did mean “manual” in reference to the Europeans!)

29

I have used engine braking on a diesel, but I do not know if it utilized manual fuel injection (it was a diesel Isuzu pickup truck).

I do not know if it was a fluke, but I will say that as a mechanic, we did see a few cars come in which had either jumped or broken rubber timing belts as a result of what the driver described as engine braking gone wrong (too violent an application of it). One time was a Honda which was using engine braking up an off ramp when the cam drive sheared off all the teeth on the belt, and another was a downshift to slow for a stoplight, where they went from 4th to 2nd and over-reved it, snapping the belt. Then it was “Pistons vs. Valves - this time, it’s personal.” :wink:

30

Diesels most assuredly do have engine braking. There is no real difference in the basic physics from a gasoline engine. In both cases, the fuel is almost completely shut off. The gasoline engine has the added braking of having to intake against a partial vacuum from the closed throttle plate but the diesel has added losses from the greater compression ratio which will increase the losses in the compression/expansion cycle (absent the Jake Brake, of course, which makes the loss 100%). Having owned both diesel and gas vehicles, I can’t say I’ve noticed any real difference between them.

That said, you folks are confusing me a bit. I believe the diesels do, for all intents and purposes, shut the fuel off completely as long as RPM is above the throttle set value. And Una, what would “manual fuel injection” be? Did you mean “mechanical fuel injection”? The controls are different but the results are equivalent.

31

I’ve driven manual transmissions for all 17 years of my driving history, and I routinely downshift to slow the vehicle.

The way I drive, I believe the hardest hit piece of the drive train is the U-joints in the drive shaft. I’ve replaced enough of these to think they’re the weak link.

You start to feel them go bad early on, as you let off the accelerator suddenly and the car lurches ahead for a moment while the slack in the driveline catches up and the wheel rotation then is suddenly slowed by the remaining inertia from the rotating engine…

(Maybe not the most elegant explanation.)

32

That was me thinking on two things at once and typing the wrong thing. I was responding to someone else on synchromesh and how it works elsewhere, and typed “manual” instead of “mechanical” here. It was late… :o in any event, I only noted it was mechanical (typed as “manual”) because the previous poster Mr. Woodall noted it. I cannot see why a diesel engine would not have tremendous engine braking capability, and I have seen the results of trying to push start a diesel truck, which gives some nice visual representation of trying to fight that 21:1 compression (it’s rather funny, unless it’s your truck…).

33

<hijack>
Let me expand a bit on the engine braking of diesels. I have been paging through my old (20 years old!) notes and books from University days, but have not been able to find the particular reference, that’s why I hope some Mechanical Engineer reads this thread and can give us more definite answers. Google also turns up zilch.

First of all, of course diesel-engined vehicles do have engine-braking capabilities, but it is not something that is inherent in a diesel engine design. They sort of have to be forced to use themselves as brakes.

Here’s why (where are you mr Mech. Engineer?!):
(1) A diesel’s rev’s depend on the fuel distribution valve setting and the current revolutions of the engine (higher revs would deliver more fuel) Totally independent of how much air reaches the engine (which is what you need for the engine to 'pump’so that it brakes)
(2) Opening the valve by pressing the accelerator would deliver more fuel which would make the engine go faster. Fine.
(3) Lifting one’s foot off the accelerator would not really affect the business, because the engine’s current revs also determine how much fuel is allowed through the distribution pump.
(4) In fact, a diesel engine without the proper safegaurds, would, upon starting, accelerate indefinitely, opening up the valve more and more, purely because of the revs already reached until the point where the engine self-destructs or the fuel runs out or the fuel pump reaches its maximum delivery rate. This is sometimes known as overfueling - especially if one tinkers with the mechanism to deliberately cause a certain amount of overfueling. Diesel engine manufacturers usually expressly waive the warranty where overfueling is evident.
(5) This means that a diesel engine would normally be unable to provide engine braking - the rising revs would actually just deliver more fuel which would negate the lifting of the accelerator.
(6) Manufacturers therefore build in a mechanism to stop the diesel supply completely or almost completely when one lifts one’s foot off the accelerator. THEN the engine can actually act like a very high compression piston pump, and provide very good engine braking.

Petrol engines depend on the throttle butterfly valve, which controls the airflow, and the airflow controls the amount of petrol mixed in the air-fuel mixture. Therefore, “throttling” the engine would limit the airflow, which limits the fuel, which limits the revs, which provides engine-braking. Easy.

Capisce?

Now, I’m waiting for that Ph D (Eng) (Mech) to tell me I’m talking complete hogwash, but that’s what I remember. BTW, I’ve been talking mechanical/carburettor here, not electronic fuel injection, which might use different techniques. The underlying principle is the same though.

<hijack of the hijack>
Did you know that diesels actually cool down while idling? Unlike petrol engines which heat up.
</hijack of the hijack>
</hijack>

Back to the OP: Engine braking if used moderately and in conjunction with your normal brakes would not harm your car in any detectable way over the lifespan of your car.

Regards,
Dries Venter

34

‘gears to go, brakes to slow’ is what the modern generation of driving instructors ‘are’ telling students. Engine braking dates from the days when most brakes were drums, not discs, and weren’t servo-assisted, and would fade on you in a tight spot. Like feeding the wheel though your hands, it’s not really necessary on modern cars. Some people have never updated their driving skills since they got their licence, and persist in thinking they should still be driving like they did in the 1950s.

35

sure replacing the clutch is expensive compared to brakes, but unless you truly suck working the clutch you will still have to replace brakes 2-4times more often than the clutch.

swapped out one clutch in 8 years with the same car, in that time frame I went through at least 3 sets of front brakes and one set of rear. car died due to no related (to the clutch) reasons but I have no reason to think that it woulndt have lasted another 5 or 6 years.

I always downshift to slow, and I drove that old car hard. I learned to 4 wheel drift, learned about horizons when driving at speed, how to use gears to force a controlled skid on tight corners. in short I beat the hell out of the clutch in that thing.

short version the clutch argument is just as dumb as the engine braking argument. neither one will really do your car any harm, drive however you wanna drive and forget about it.

36

DOH! stupid mis tab.

my current car is an automatic, with full time overdrive (unless you hit the button) and I have recently been forced to punch it hard and take the shoulder to avoid getting rear ended. that stupid overdrive nearly got me hit. (well really it was the unattentive asshat behind me but…) even without the OD automatics often lag between the time you hit the gas and the time the car starts to pick up speed, sure its usually less than a second but it drives me nuts. to the point where I am definitly not getting an automatic next time.

the control factor is definitly there, just like people who prefer Rear wheel drive over front for the extra control allowed. unless you take the time to learn how to really drive you will never even know its there.

37

To expand on Una Persson’s discussion of vehicle damage from push starting or roll starting a diesel:
I had a dead battery in my Benz. I let my old diesel Benz roll down a hill untill it was at 15 MPH. I dropped it into first gear with the ignition turned on. The vehicle skidded as if I was flooring the brake pedal, the engine turned over twice, and the car stopped. My friend was in the passenger seat. One of the bolts securing said passenger seat sheared at this point, neccessitating repairs if the occupant of said seat didn’t want to assume a kinda’ sleeping posture during driving.
Good times. Never gonna’ try that again though.

38

My 2 cents:
Believer in “standard” gear boxes and double-clutching which I practice every day (smoothen the ride, and I can hear my V8 :wink: ) I shouldn’t have any problems according to the upper conversation.
I believe in ear-shifting too (synchro rev and no clutching like ** venterap ** : good training ! )

I was told that engine braking was good because the high rev would put more oil on top of the engine. I always had my doubts. Any input?

And I use to have a small 1.9 liter peugeot 205 diesel that I would start very often on 3rd gear in a down slope. Entertaining.

39

OK, I don’t have my PhD in Mech Eng, only my MS, but I think I can still respond to venterap’s question. Of course, a diesel will not have engine braking if the fuel metering system fails. In the same manner, a gas engine will not have engine braking if the throttle valve sticks open. That is not exactly unheard of either. (I’ve got a great story about the throttle slides sticking open on a Harley Davidson that was being pulled to start it. Not much engine braking in that ugly event.) Neither of these events seem to me to have much to do with the “inherent design of the engine”.

Push and pull starting diesel vehicles can be done if care is taken. Basically, you slip the clutch. As an interesting aside, some two stroke diesels can be push started with the transmission in reverse and the engine will run backwards.

More oil on top of the engine at high revs? Not on any correctly designed engine.

40

Critical1, what kind of engine? I remember that back in 1995 while waiting to pick up my brand new Honda Civix EX manual, they gave me a '95 Honda Civic EX automatic (they wanted my trade-in that day for some reason). It sucked juevos. Stamping on the damn pedal took almost a second for downshifting and acceleration to start. But that’s a tiny 1.6L engine. Every non-4-cylinder car I’ve had since then hasn’t had that reaction problem. My two V6 Bonnevilles and my current V8 Continental react virtually instantly. I had a 4-banger Ranger for a bit, but it was back to the slow reaction time on the tranny.

I’m not saying that it’s particularly the 4-cylinder that’s the problem; it could be the design. Maybe for some reason the engineers decided to pair a crappy transmission to 4-cylinder engines. Or it’s the engine. I don’t know. I’m just saying that the 4-bangers react crappily, and the V6+ have very good response.

Oh, that manual Honda? I did downshift, mostly for fun. Not for any need nor conserving of the brakes. I don’t think I ever put brakes on it. The previous Honda I did the same, but I put brakes on it for sitting two years (do a thread search). In reality, if it’s not for fun, it’s a hell of a lot easier to stomp on the brakes than downshift. Don’t forget that downshifting doesn’t activate your tail lights! Or is that part of the fun?