Like, what is the mechanism that slows the car considering the hydraulic brakes aren’t involved at all? How does it vary by transmission type (manual, regular auto, CVT)?
In: 14
It’s all in the engine. The gear/transmission method doesn’t matter at all, though something with manual control can be good to force lower gears to get a stronger engine braking effect. Most automatics try to shift to a higher gear to minimize engine braking and letting you get the most coasting you can. A full manual, or an automatic with a manual selection mode, helps immensely.
In a typical gas engine, without fuel, there is very little air going into the intake of the cylinder, so as the cylinder moves down on the “intake” stroke there’s a near-vacuum on it. The cylinder goes through the compression+combustion strokes without accomplishing anything since there’s no fuel, then the “exhaust” stroke tries pushing upwards on the cylinder to force out all the burnt fuel. There is none, but this step is done against a full pressure air source on the exhaust pipe. So vacuum on the intake, pressure on the exhaust, is a net negative pressure on the engine making it slow down.
A diesel engine is built differently, so this strategy doesn’t work. Instead there’s what’s called “Jake brakes”, which has a mechanism open the exhaust valve at the start of the “combustion” phase. At this moment the engine has spent energy compressing the air inside the cylinder (with no fuel) which took some decent energy. Normally the “combustion” stroke would have the cylinder move back again, decompressing the air and balancing out the energy loss… but instead, we open the exhaust valve and let the air out immediately in a burst of compressed air so it can’t push the cylinder down again. That’s why trucks doing engine braking are making this loud noise. But it works.
So we just connect the engine to the wheels in spite of the fact we don’t want engine power, and let the engine braking effect slow us down instead.
Latest Answers