Inspired by a discussion on engine braking in this thread:
While heat transfer is an inevitable part of engine braking, it’s possible to envision a hypothetical engine with no heat transfer at all that still provides engine braking.
Consider an ordinary gasoline four-stroke engine. In the intake tract, there’s a movable throttle plate: open it wide, and the engine produces maximum torque, close it down to almost nothing and the engine breathes in just enough air (and fuel) to overcome friction and keep the engine idling at low RPM.
Now point the car downhill, close the throttle to its idle position, and shut off the fuel. Gravity keeps the car moving downhill, which keeps the engine spinning. During an intake stroke, the engine pulls air from the intake manifold, lowering the manifold pressure to about 0.3 atmospheres, and the pressure in the cylinder (which is open to the intake manifold) is also about 0.3 atmospheres. Meanwhile, the pressure under the piston, down in the oil sump, is still about 1 atmosphere. So the air in the cylinder does work pushing the piston down, but the piston does about 3.3 times as much work pushing the atmosphere down into the sump. This is where the engine braking effect comes from on a throttled gasoline engine. Disregarding heat transfer, the compression and expansion strokes cancel each other out: the piston does work on the trapped gas during compression, and the trapped gas gives that work back to the piston during expansion. At the end of the expansion stroke, the cylinder is back to about 0.3 atmospheres - but then the exhaust valve opens, and now the cylinder is at 1 atmosphere during the entire exhaust stroke. The net force on the piston during the expansion stroke is zero, because there’s the same pressure on the top and bottom of the piston.
I ran through some math on this, and for a naturally-aspirated gasoline engine, I figure the above phenomenon can provide about 5% of the engine’s rated torque as a braking effect. Add in the heat transfer that you get during the compression and expansion strokes, and that figure goes up a few more percent. Add in the actual mechanical friction of the moving machinery in the engine, and it’s maybe another five to ten percent. All in all, you might be able to get an engine braking effect on your gasoline-powered car that’s about 15-20% of the engine’s peak rated torque.
Since the braking effect from gas dynamics is due to the throttle plate being closed during the intake stroke, if you open the throttle plate up, you pretty much eliminate that aspect of engine braking (though you’ll still have the engine’s mechanical friction and heat transfer effects). I’ve demonstrated this to my friends with my motorcycle: disconnect the fuel injectors, hit the starter, and it cranks per normal; twist the throttle wide open, and now the starter spins the engine faster because the engine is no longer trying to breathe through the equivalent of a pinched straw.
This also describes a diesel engine, which has no throttle plate at all. If you disregard heat transfer and mechanical friction, a diesel engine doesn’t provide any engine braking:
- atmospheric pressure in the intake manifold means atmospheric pressure in the cylinder during the intake stroke. No work done here.
- The work done during the compression stroke is given back to the engine during the expansion stroke, so they cancel each other out: no net work done here.
- Atmospheric pressure in the exhaust manifold means atmospheric pressure in the cylinder during the exhaust stroke. No work done here.
In reality, you do have the mechanical friction of the engine and the heat transfer during the compression expansion strokes, so you do get some engine braking. Because diesels run at higher compression than gasoline engines (~16:1 for a diesel versus ~10:1 for a gasoline engine), the peak temps at the end of the compression stroke are higher, and so there’s more heat transfer to the engine parts, and a correspondingly greater braking effect from this particular phenomenon.
The special trick you find on big over-the-road diesel engines (typically just the ~15-liter monsters that go in highway trucks) is compression release brakes (of which Jake Brakes are the most well-known brand). When the piston is at top dead-center after compression, just when cylinder pressure is the highest and the most mechanical work has been extracted from the crankshaft, the mechanism briefly opens the exhaust valve, allowing cylinder pressure to come down to (or close to) atmospheric. Now all the work that was extracted from the crankshaft during the compression stroke is pissed away as a stream of high-velocity gas in the exhaust manifold. Not only that, but now the cylinder pulls a vacuum during the expansion stroke, resulting in a bit more work being extracted from the crank. At BDC, the exhaust valve opens, returning the cylinder to atmospheric pressure for a zero-net-work exhaust stroke.
Disregarding heat transfer and engine friction, by my calcs the compression release brake system can allow a diesel engine to develop a braking torque equal to about 30% of its rated output torque; add in the actual mechanical friction and flow head losses in the intake and exhaust tracts, and losses due to heat transfer during the compression and expansion strokes, and the number gets quite a bit higher. If Wikipedia is to be believed, total engine-braking torque when operating at high RPM can actually be on par with an engine’s rated torque output.
As far as temps go, well, for gasoline or diesel, it’s not running any hotter than it is when it’s idling, because it’s doing pretty much the same thing as far as squeezing/expanding the air in the cylinders. In fact, for the case of the diesel using a compression-release brake, temps are actually very cool during the expansion stroke. Likewise, the loads on the machinery aren’t particularly large, unless you’re spinning the engine at ludicrous speeds.
Is engine braking hard on the gears in your transmission/gearbox? Not particularly. The modest deceleration of engine braking is no harder on the gears than accelerating at the same rate. The faces of gear teeth are extremely hard and will probably be the last things to fail. More to the point, the “front” face of each gear tooth is what racks up the wear as you drive 100,000 miles in the forward direction; the “rear” faces of the teeth, the faces that get loaded during engine braking, will only see a tiny fraction of the miles that the “front” faces do.
Is engine braking hard on the clutch(es) in your transmission/gearbox? If you’re a skilled manual transmission enthusiast, then when you downshift, you can rev-match the engine before you let the clutch out, resulting in minimal clutch wear. For automatic transmissions before the advent of electronic throttle control, downshifting meant that clutch bands inside the transmission had to briefly slip to drag the engine up to speed during a downshift, and yes, that meant wear. For at least some late-model cars, that’s a thing of the past: when you request a downshift, the computers that are actually managing your drivetrain for you will blip the throttle to rev match, resulting in virtually zero wear on the clutch bands in the automatic transmission (or clutch plates in your dual-clutch transmission).
Should you use engine braking? If you’re driving an 80,000-pound tractor-trailer rig in hilly or mountainous terrain, you will probably die if you don’t, because you’ll fry your brakes. For big pick-em-up trucks towing big fifth-wheel trailers in similar terrain, it’s probably also advisable (check your owner manual). For cars? if you’re heavily loaded (4-5 people with luggage) and descending a long mountain grade, it might help you avoid warping your brake rotors. For anything less than that, it’s more of a convenience, to keep you from having to jam on the brakes as often. I do it on even on modest hills in our neighborhood
