Over-reving a diesel

My wife drives a 99 New Beetle w/ a TDI engine. We have a dissagreement regarding the use of the engines compression to slow the car down on the long and steep grade below our house. I’ve suggested using first gear and holding the rpm’s under 2500. She lets the tach go up to 3500 and possibly higher in spite of my pleading to go easy on the engine. Any thoughts?

will do no harm at all!!this engine is designed to operate up to 4500rpm,and has a rev limiter to prevent damage,but why not use the brakes???

Because diesels don’t have throttles to restict air flow through the engine, diesels have very little engine braking to begin with. Semis use exhaust throttles and/or Jake brakes to give their diesel engines some engine braking.

A rev limiter will not prevent overrevving through agressive downshifting.

I also have a TDI and while the two prior responses are correct, I will try to put them together and end up more correct.

First your diesel has no throttle, so it has far less engine braking than a gasoline engine. While it is not likely to damage the engine at 3500, I suggest that in this case you may want to use some brake along with the engine braking. It will give you better control and it is very unlikely to overheat the brakes.

If you over do it, you can slip the timing belt and if you go more than one or two teeth it can cause very expensive engine damage.

I suggest a little compromise in this case.

Small diesels having less engine braking (I can’t speak for a large diesel) is the theory but I note no noticable difference when alternating between my diesel and our four cylinder gasoline car, both with manual transmissions. The diesel car does indeed have a fair amount of engine braking. Possibly some of the reason for this is that the gasoline car’s gear ratios are taller.

In any case, yes, use engine braking and supplement this with the brakes but also, as was said, don’t let the engine overrev beyond the maximum permissible.

Gently is best but you live where you live and a diesel should handle the RPM’s ok. Lots of diesels run better if you let em rev on acceleration. I don’t see harm coming to it by letting it run to 3500 RPM’s when decelerating.

A lot of gas cars are also using variable cam timing that puts the intake cam in a low intake vacuum phase during idle and part throttle use. This reduces both engine braking and part throttle/idle fuel consumption.

Because diesels aren’t fighting that high intake manifold vacuum during idle, their idle fuel consumption is only a fraction of a comparable gas engine’s idle fuel consumption. It’s also why diesels need an idle governor. On a gas engine, anything that slows down the idling engine causes the intake vacuum to drop which in turn causes the cylinders to fill with more air/fuel mixture which causes the engine to automatically make more torque to counteract the increased load.

here in england,my works van is a 2003 vauxhall corsa with an isuzu sourced 1.7 litre turbo-diesel engine,and its red line is a high(for a diesel) 5000rpm.i regularly drive upto 4800-5000rpm in town stop and start driving,and its done no harm for the last 5 years!!and i get 38 mpg in town driving,no doubt i would get more mpg if i didnt rev it as high!i think vw 1.9 tdi s red line is lower,about 4500rpm,but it will do no harm to go up to that,so dont worry!

Thank you all for your responses. If nothing else, I’m feeling a little more comfortable w/ the higher RPMs. We drove down the grade together and reached a compromise I can live with. So great to have this resource…thanks again.
Fiddle Al

BRAKES are so much cheaper than engines…Why flog the poor engine?

Also, there is no such thing as “compression” braking. The drag you feel are the inertia loses, the load caused by the reciprocating parts starting and stopping, not compression…That load is ALWAYS there…When you are driving along, it’s overcome by the engines POWER…The higher the RPM, the greater this load becomes, greatly reducing the efficiency of the engine.

Well in a gasoline car there is very real engine braking related to compression. The engine is really an air pump. When the throttle is closed little air can come in so the pump has to work harder and it gets its energy by pumping that air, which will compress it.

A diesel has no throttle so its braking power is far less, unless it has a special valve to reduce that air intake.

There are no inertia losses in a piston engine. The energy needed to accelerate the piston is returned to the crankshaft when the piston is decelerated at the end of the stroke. On in-line fours where all pistons stop at the same time, this results in a cyclic speed variation of the flywheel, the flywheel speeds up a little as the pistons reach top and bottom dead center and then slows down a little as the pistons re accelerate towards the middle of the stroke.
On six and eight cylinder engines, there is always a piston decelerating while another is accelerating and the energy just goes from one piston to another.

During WWI, a lot of airplanes were powered by 9 cylinder radial engines that had a stationary crank that was bolted to the plane’s firewall and the entire crankcase just rotated around the crankshaft. This resulted in the pistons just going around in a circle centered around the crank throw. These engines were no more efficient than conventional engines.

The pendulums of old fashioned weight driven clocks also reciprocate, kinetic energy is turned into potential energy and then back into kinetic energy over and over and over. Yet, these clocks usually could run for eight days on a single winding and some could go for 31 days.

In a gasoline engine, throttle closed, there is NOTHING to compress…In a Diesel the power required to compress the air is returned on the downstroke…

Inertia losses can be demonstrated by removing the cylinder heads from an engine and spinning the engine with an electric motor…As RPM increases, the amount of power needed to spin the engine increases dramatically…Pistons are not pendulums swinging in an arc…It takes tremendous energy to start and stop reciprocating mass moving at high speed…These are the forces you feel when you downshift and “brake on compression”… The higher the RPM, the greater these forces become and it has NOTHING to do with compression, because there is little in the engine to compress…

“In a gasoline engine, throttle closed, there is NOTHING to compress”

The throttle is never completely closed. There is air for idle, through the throttle and the idle air passage/valve.

“It takes tremendous energy to start and stop reciprocating mass moving at high speed”

And where, prey tell, does all this energy end up? Something would get mighty hot (even without burning fuel) if it was as you say.

If you could take the head off an engine and just spin the bottom end (crank and pistons you would find it takes little energy.

“If you could take the head off an engine and just spin the bottom end (crank and pistons you would find it takes little energy.” Wrong! As RPM increases, the amount of power required increases very quickly…There is little friction, so there is little heat produced. The energy is CONSUMED accelerating and then decelerating the reciprocating mass.

At some point on the RPM scale, inertia losses equal the power available and available horsepower drops to zero. In real world engines, these inertia loads exceed the structural strength of the engine and it flys apart before inertia loads prevent further RPM gains…

These laws of physics are why racing engines do everything possible to reduce reciprocating mass. In this manner, they allow the engine to operate at RPM levels it otherwise could not reach…

Compression braking applies to diesels only. Because the throttle is always open, the cylinders get filled with air and it does get compressed.

In a gasoline engine, the throttle is closed, so the intake manifold builds up a high vacuum. When the intake valve opens, the piston, which is on the down stroke, is subjected to a lot of suction and that is where the engine braking comes from. The vacuum in the intake manifold is trying to keep the engine from turning.

Since there is so little air in the intake manifold, the compression stroke is essentially compressing a vacuum. I doubt that even at the top of the compression stroke that the pressure in the cylinder reaches even one bar.

The same suction that held back the piston on the intake stroke sucks the piston back up on the compression stroke…Little energy is lost…In a Diesel, just the opposite happens with the same result. Energy expended on the compression stroke is recovered on the power stroke…

If you installed a headless engine in a car and rolled it down a steep grade and the driver downshifted to increase the RPM, the same “engine braking” effect would be felt. It takes energy to accelerate mass. The reciprocating mass of the engine is accelerated to high speed and brought to a stop twice each revolution of the engine. At TDC and BDC the piston is motionless. At 90BTDC and 90ATDC the piston is traveling at its highest speed, depending on RPM. You can not accelerate a piston to 3000 feet per second speed without expending tremendous energy. To slow the piston down to zero FPS requires an equal amount energy. This energy is dissipated in trying to tear the engine apart, not as heat. That’s why high-speed reciprocating piston engines can not do better than 30% efficiency, that and the heat loss…

You have energy confused with force. It takes a lot of force to reverse the motion of a piston, not a lot of energy. The peak force is at top dead center when the piston speed is zero. The peak speed happens when the force is zero.

energy is force times distance
power is force times speed.

All the force in the world times zero speed = zero
All the speed in the world times zero force = zero

Does it take considerable power to motor a headless engine at high speed? Have you actually tried it and measured the power? Have you done it with the engine in a vacuum chamber to eliminate air pumping losses?
Don’t underestimate how much power it takes to push air around at high speeds. Air is not weightless, a cubic yard of it weighs about two pounds at sea level.

A lot of formula one engines have vacuum pumps to get rid of the air in the crankcase to eliminate the pumping losses from the bottom of the pistons and crankshaft air resistance, it is claimed that there is about a 30 or more horsepower gain by doing this.

If you really want to see a headless engine do some braking, submerge the engine in an oil bath.

The reason heat engines are limited to about a 30~35 % thermodynamic efficiency has a lot more to do with entrophy than it has to do with mechanical efficiency.

“The same suction that held back the piston on the intake stroke sucks the piston back up on the compression stroke…Little energy is lost.”

This statement is true as far as it goes but let’s go farther. There is still vacuum in the cylinder at the end of the power stroke and then the exhaust valve opens and the vacuum sucks exhaust air back into the cylinder in what amounts to an implosion. Now the vacuum is gone and the piston has to push a cylinder full of air back out the exhaust during the nest upstroke, Voila! pumping losses and engine braking.
Have you ever stopped and wondered why engines are loud during overrun with the throttle closed? Suddenly releasing vacuum makes just as much noise as suddenly releasing pressure. Surely you have noticed that an un-muffled engine has a quiet zone at a certain throttle opening, that’s the crossover point where vacuum changes to pressure.

No matter what I ask my wife to do when it comes to an automobile . . . she seems to do the opposite. I just live with it, fix things as they need fixing and remind her when she is doing something I consider “wrong”. If she blows the engine it is still cheaper than ruining your marriage and health bickering about it. You’ve told her what you think about the over-revving, now move on. I’m a car-nut and love talking/thinking/tinkering/ with cars . . . but some folks (my wife) aren’t . . . and you’ll live longer if you just accept them as they are. Turn up the radio when she’s driving. Rocketman