Engine Brake Question

I dunno about burning up valves..never had it happen on a company truck or otherwise but.in actual highway driving on the interstate...how often do you actually hit the brakes or use a jake..in reality its not much..more often than not you flip off the cruise and ease out of the throttle because you don't have to slow down too often or too much..

Petey

 

Actually, no. A diesel engine is unrestricted on the intake and there is no fuel mixed into the intake charge. It will breathe in as much air as it can get past the intake valve on each and every intake stroke, without regard to throttle position. Energy is consumed (or stored, if you will) as the piston compresses the air in the cylinder during the compression stroke, and when the cylinder nears top dead center on the compression stroke, one of three things will happen:

1) If you have your foot on the throttle, fuel will be injected into the cylinder where the heat generated by compressing the air ignites the fuel. The fuel burns, heating the compressed air and causing it to expand...increasing the internal pressure in the cylinder and pushing the piston back down the cylinder with significantly more force than what it took to compress the air.

2) If you do NOT have your foot on the throttle, and the compression release brake is turned OFF, nothing special happens. The compressed air in the cylinder acts like a spring, and pushes the piston back down the cylinder with almost as much force as it took to compress the air. The "braking" force you feel is the drag...the friction between the moving parts in the engine, transmission, and the rest of the drive train along with the rolling resistance of the truck & trailer on the road.

3) If you do NOT have your foot on the throttle, and the compression release brake is turned ON, then when the piston approaches top dead center on the compression stroke, the exhaust valve opens which allows the compressed air in the cylinder to be released into the exhaust. The "braking" force you feel is the air being compressed in each cylinder, which because the air is then released, it does not act as a spring to push the piston back down the cylinder....so the resistance of the compression strokes is felt as a braking force through the drive train.

In other words, the "energy" the engine brake uses comes from the rotation of the crank moving the pistons to compress the air in the cylinders. Rather than the stored energy (compressed air) pushing the cylinder back down, it is released through the exhaust valve.

Now as for Allow Me's comment about burnt valves, if the compression release brakes are functioning PROPERLY, it shouldn't happen....no fuel is being injected into the cylinders while the compression brakes are opening the exhaust valves at TDC, so no combustion is taking place. However, if for some reason the exhaust valve is opening at the top of the compression stroke while fuel is being injected into the cylinder, then I would imagine it would be possible to burn a valve since the combustion is taking place as the gasses pass the valve into the exhaust manifold rather than being contained within the cylinder.

 

I was going to bring up the "burnt valves" wives tale when PBD was trying to sell the "reduced driveline life" story but decided against it. If someone wants to believe that Jakes burn up exhaust valves or that they reduce the service life of driveline components, I'm fine with that. I've never seen evidence to support either claim and therefor am not concerned about either one. If someone else is worried about it, then they should drive their truck accordingly. In the end we are all responsible for our own decisions and if I have to change a U-joint @ 800,000 miles......OK. Its part of the freedom of owning your own equipment.

 

Nicely put.

 

I second that

Petey

 

Glad to hear somethings are going down.....there's 3 or 4 shops on 10 / Houston shouting on the radio selling all new shoes ( drives ) and hardware for 3 to 400 installed. Granted that's not a full meal deal like Petey got but that's a long way from 8 grand and 7 years...... yes?

 

Licensed to Kill is correct...you are not.

Sorry...

What would you have done 30 years ago when few trucks were equipped with Jakes?