Quote:
Originally Posted by maxnix
I suggest you do that! Maybe this time you will get it! Good luck!
Have you heard about lifetime sealed automatic transmissions?  |
originally posted by bradleyland
Quote:
Everything is worn in reverse. However, if you've been inside the mechanics of a car, you know that nothing in the driveline is built to operate in one direction. The gears, differential, etc are far more sensitive to shock (in either direction) than they are constant pressure. You could run your gearbox for 200k miles in either direction (forward or backward) under constant design-load pressure and it wouldn't wear out.
Let's talk about the kinetic energy part of your view, because it's a really good thing to understand. Brakes slow your car through friction. Friction generates heat. The heat is dissipated as radiant energy and and convective heat transfer. Much of this is dissipated in to the air, as well as soaking in to wheels and suspension parts attached to the wheel carriers. The problem is, air is a bad conductor of heat, so this process is relatively inefficient. When your brakes fail to brake any more, it is because the heat soak is so pervasive that no more heat energy can be absorbed.
Under compression braking, a different type of kinetic dissipation occurs. Your engine is a reciprocating assembly. It is engineered to:
* go round and round without wearing out
* convert heat energy created in the combustion chamber in to kinetic (motion) energy
* direct the force created from the energy-to-motion transfer in the most efficient means possible
* dissipate wasted energy through the exhaust and a water-to-air heat exchanger (radiator)
* BONUS FOR TURBO: harness wasted heat energy from the exhaust to boost output
The wear surfaces on rods, pistons, cranks, crank journals, etc are all cylindrical. They wear in a somewhat oblong pattern due to the forces involved with a piston that cycles back-and-forth a thousands of times a second, and in a well lubricated environment, they wear very little. This pattern doesn't change significantly whether you're accelerating or decelerating. I'm not talking out my ass here. I've held these parts in my hand and inspected them for wear while (sometimes) replacing them.
When you use compression braking, the kinetic energy from your forward momentum is transfered to heat energy by compressing air in the combustion chamber. Modern engine control systems do not inject fuel under decelerating conditions, so there is no ignition occurring in the cylinders. Your engine is (literally) perfectly designed to manage this type of energy, and it does so in a far more efficient manner than your brakes. As a matter of fact, your brakes are the least efficient mechanism on your car. Their sole purpose is to waste energy every time they are used.
Assuming that compression braking is "bad" for your car in a way that normal operation is not is based on an incomplete understanding of the systems that make up your driveline. Your engine, gearbox, and rear-axle assembly are all perfectly capable of operating for hundreds of thousands of miles in either direction. There is no reason avoid engine braking, provided you can do it smoothly. It is not a free lunch, but it is arguably a less expensive lunch than using your quick-wearing brakes.
|
1 thing i'll add-
i've driven a n55 dct, and i'm currently driving a step 328i loaner vehicle.....
these transmissions downshift by themselves, without me even touching the brakes. how can downshifting be bad for a car's engine or transmission when BMW factory automatics downshift on their own?
maxnix-
