I have a manual transmission car, with an incredibly smooth gasoline I6. It’s great fun to downshift and use engine braking to slow the car, because even at 5000+ RPMs, there’s no vibration at all, just a beautiful turbine-like wail.
Anyway, I recently started to wonder what actually provides the braking force when I do this. I don’t think it’s just friction between engine components, because I’d imagine the engine would wear out quite quickly if that was the case. But I can’t think of many other possible sources - when the throttle is closed, there’s no air coming into the cylinders, and so there’s nothing to compress. I suppose creating that vacuum on the intake stroke would require a bit of force, but wouldn’t the pressure of the atmosphere just push the piston back up on the compression stroke?
This is wrong. The throttle is designed so that it allows a small amount of air to pass when fully closed. If the throttle made a perfect seal, then the engine would stall whenever you released the gas pedal.
But I don’t understand how compression of the air would provide much braking force either. Wouldn’t the cylinder act like a spring, with most of the force used to compress the air on the compression stroke directed back at the piston on the power stroke?
Is it simply the intake of air that provides the force? That doesn’t seem like it would be adequate either.
Maybe a better way to put it would be without applying throttle there is no real push during the power stroke so the work you used during the compression stroke generated no return.
A good part of the compression energy is converted to heat and dissipated through the cylinder walls.
You can simulate the same effect using a syringe (without a needle!). Block the tip with your finger. Depress the piston as far as you can and then release it. You will notice that the piston will not return to its original position.
The other part of the engine braking force comes from the restriction posed by the throttle during the intake stroke. Diesel engines do not have a throttle, so engine braking is significantly weaker. Thus the need for Jake brakes.
BTW, I guess you realize that you are putting a lot more wear and tear on your transmission and clutch. Brakes are a lot cheaper to replace than clutches.
That’s only true if you don’t know how to downshift properly. A rev-matched, double-clutched downshift results in more wear on the sole of your left shoe than the clutch and transmission.
The “air spring” thought works for compression and “power” strokes, but NOT on intake and exhaust strokes. During compression braking, the engine is operating as a vacuum pump. It is pumping the low pressure intake manifold gas up to the pressure in the exhaust manifold (ambient).
This pumping loss exists to some degree any time a spark ignition engine is operating at less than wide open throttle. This is the main reason that large gasoline engines exhibit poor fuel economy when lightly loaded, but don’t do much worse under heavy load (say pulling a trailer)
Because diesels do not suffer such pumping losses, thier fuel consumption is much more closely related to load. And as mentioned, they do not produce much engine braking. In addition to “jake brakes” a valve restricting the exhaust can be added to improve engine braking performance.
True. But I don’t understand why anyone would want to slow their car down with engine braking rather than using the brake. The only time I would use engine braking is when going down a hill. I can’t think of any situation where engine breaking would provide an advantage. Normally, if I’m double de-clutching or, even better, heel-and-toeing, then there is some braking involved anyway.
No advantage, it’s just fun. But actually, usually I do it only when I need to slown down for a corner, off-ramp, or turn onto a side street. The engine braking is just a side benefit - the real purpose is to be in the correct gear for the corner.