September 16th, 2007


jake brakes

ey wow, I have a LiveJournal.

I spent my home time organizing the house and spending time with welfy mostly. I've been back on the road for a few days now. I've acquired a new student, named Thomas. He has a horrible cold and is constanlty coughing. Today the back of my throat has started feeling sore. Lovely. Other than that, he's an alright guy. He's doing well and we get along; can't ask for much more than that.

Since autumn officially starts next Saturday, I'm going to try and finish up posting gams photos this week since it's supposed to be a summer project. A few more have straggled in over the past couple of weeks. If you've been putting it off, this is your last chance.

A small discussion began in my last entry about what "jake brakes" were. Rather than reply, I thought I'd just write one of my little truckgeek explanations for the remainder of this journal entry, just in case you ever wondered. This is a really basic explanation of somewhat complex mechanics and thermodynamics. But here's the meat of it.

The Jacobs Brake is actually a brand name which has become synonymous with all engine compression brakes. Diesel engines don't work like gasoline engines at a very fundamental level, even though both are four-stroke engines (one powered stroke in four). In a gas engine, you need fire, fuel, and air. (1) Air is throttled at a precise ratio with the fuel, mixed, and sucked into a cylinder by a piston on a down-stroke. (2) An up-stroke compresses the air/fuel then it is ignited by the spark plug, (3) causing an explosion which provides a powered down-stroke. Finally, (4) a small valve opens in the top of the cylinder and the smoke/heat is exhausted during the final up-stroke. The valve closes and stroke 1 begins again.

In a diesel engine, there is an absence of fire; there are no spark plugs. (1) Unthrottled air is sucked into the cylinder and (2) compressed. In a diesel engine, the compression ratios are much, much higher, creating temperatures upwards of 1,500 Farenheit as the air is squeezed. In other words, there's no need for a spark plug. (3) Fuel is then injected into the hot air causing ignition for the powered down-stroke and (4) the final up stroke exhausts the fumes through an opened valve.

If you've ever stuck your tongue in the top of a soda bottle and then sucked all of the air out using your tongue as a valve, you know the force the vacuum can apply as it tries to pull your face into the bottle. In a gasoline engine, this is what happens when you take your foot off the gas. Stroke 4 just pushed everything out through the exhaust valve. Since you're not giving it gas, nothing is sucked into the cylinder, so stroke 1 creates a vacuum inside the cylinder. This causes drag on the engine in stroke 1 and stroke 3, as the vacuum tries to pull the piston back up during these two down strokes.

Since air going into the cylinder of a diesel engine is unthrottled, this doesn't happen. Even if you're not supplying fuel, the air still is tightly compressed on stroke 2, but then acts like a spring pushing back down on the piston for stroke 3, the power stroke. In the case of a heavy truck going down a hill, gravity becomes "free energy" from the drive-wheels, through the transmission, providing compression only to have it push the piston back down in a powered stroke and send the energy back to the drive-wheels where it's added to the gravity... again.

What someone figured out was that by adding a second exhaust valve, which could be opened at end of the power stroke (3), the hot, compressed air could be exhausted. In a truck rolling down a hill, the "free energy" coming through the transmission is absorbed by the compression stroke, but is now robbed from the engine by its release through the exhaust, causing a discernible net loss in power. That is what an engine brake is and that's why it makes that loud WOP WOP WOP noise when a truck driver takes his foot off the fuel pedal; it's the tightly compressed air rapidly escaping the cylinders in succession at the end of their respective stroke 3s.

The engine brake disengages whenever the fuel or clutch pedals are pressed and can be turned-off altogether via a switch on the dashboard. They can be particularly dangerous on rain/snow/ice covered roads. Since it only provides braking power through the drive wheels, it's a recipe for a jack-knife on anything other than dry roads.

Hopefully that wasn't too confusing.