engine braking

[hudr]

Ok guys, nobody likes to have their beliefs shot down, but... I dedided to post this here for maximum exposure. Think of it as living up to what I tell my 6 year old son, if you are wrong, admit it and go on.
Anyway. I decided to do a little research. It seems a gas motor is capable of producing more engine braking due to the closure of the throttle plate.
However, a diesel is still pushing against its higher combustion chamber compression so should spin down faster, right? And I think that may be where my thinking got skewed. Any engine turning at a high RPM takes more umph to keep it spinning. In other words, cut the fuel and it spins down QUICK. And a diesel (especially non-turbo) will fall to idle pretty quickly. I remember the days of OTR Trucking and we would let the motor "spin up" on the down side of a hill to make it hold better (lower gear, more RPM) and if it still got away, hit the retarder.
I think what makes it hard to compare is the differences between most gas and diesel powered vehicles in terms of weight, transmission, final gearing and chassis. I know if I step off my 6.5 at the bottom of an off ramp I don't have to use much service brake, my mom's suburban you better grab some brake early. But a lot of that is transmission too, I bet.
So.....boils down to this: a gasser has to pull against the vacuum in the intake for the pistons to fall, a diesel doesn't....
are you listening, son?

[Nat]

A lot of the difference is in the number of rpm's it is turning. On my deisel dodge at 70 mph I'm only turning 1950 rpm's. If I let off on the throtle at 70 turning 1950 and it idles at 900 that isn't a lot of rpm's to drop, and without a butterfly closed to cause a vacuum it just crompresses the air and then the compressed air pushes the piston down as the next is building compresion. Later, Nat

[twstanley]

Exhaust brakes are put on my diesels ( Jake brake ) to improve the engine braking, from my understanding these choke off the exhaust to a certain degree which increases the engine braking greatly....and makes a lot of noise.

[hudr]

I thought an actual Jake Brake (Jacobson brake) manipulated the exhaust valves on a motor to produce braking and was different from an exhaust brake which basically blocks the exhaust pipe turning the motor into an air compressor. Anyway, I'm not here to pick on details. Wonder if its copyright law or different mechanics as to why it was called an Engine Retarder on our old Mack and a couple of newer KWs w/ Cummins power?
Someone mentioned RPM earlier. With a 5 Spd manual and 4.10 final, I turn ~2450 at 70 w/ my 95 6.5TD Chevy, so I get a whollop of slowdown when I let up. I have noticed my BIL's pickup (94 6.5TD, automatic, 3.73 final) coasts a LOT more than my manual.

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] Yes, you are correct, the Jacobs system does open the exhaust valves and bleed off compression from the compression stroke at near top dead center; thereby cancelling out the effect of the compressed air acting on the piston to drive it back down. That is completely different from placing a butterfly valve in the exhaust manifold which chokes off the exhaust to help slow the engine. [img]/forums/images/icons/tongue.gif[/img]

[hudr]

I think what really bothers me about the whole engine braking thing was that I grew up around big diesels. Farm equipment and OTR trucks. Dad was an Owner Operator nearly all his life and we did most of our own maint. and repair. We even rebuilt a couple 14 liter Cummins. I guess I just never really thought about the whole engine braking thing. Gas motors were something for Mom's car and lawnmowers. [img]/forums/images/icons/grin.gif[/img]

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] Having grown up within forty miles of the "logging country" in the Sierra-Nevada mountians, I can still remember the frequent wrecks that logging trucks had due to brake fade/burnout. When Jake brakes came out, it was like the answer to everyone's problem as to how to get DOWN the mountain and live to tell about it. We had mostly Caterpillar tractors to farm with, and I was keenly aware that the engines didn't hold back when you went downhill. You had to stand on those brakes or overspeed the engine. [img]/forums/images/icons/crazy.gif[/img]

[Pat]

Dave, When my '97 Dodge/Cummins wonton was new, I went to northern Nevada (Great Basin National whatever) carying the largest cab over slide in camper Lance made and drove up to a campsite above 10,000 feet. Coming back down I stayed in 2nd and 3rd so as to not freewheel and used the brakes as sparingly as I could and still maintain good control and keep the speed from buillding up too much. There is a stop sign at the foot of the hill where a forrest service truck was apparenty parked while a ranger type day dreamed. Luckily he moved out of the way at the last second and I ran the stop sign by 20-30 feet due to brake fade.

I contend that there was no way to make that descent with my rig and not get into brake fade. When I got back to civilization I installed the BD exhaust brake and never regreted it.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[QRTRHRS]

You are correct on the copyright. Jacobs does have the term Jake Brake registered. Next time you notice one of those road signs warning against engine brake use, check the verbage. It should say retarter, etc, never Jake Brake.

I don't have Jacob's web site handy but if you go there, they have a whole write up about it. Their argument is that using their trademark on a sign is negative to their product.

[Pat]

The best part of the educational piece on Jaobs site was the info regarding muflers and how it is mostly poor or missing mufflers that allows a retarder to make a lot of noise and not something inherent in the retarding technology.

One of my pet peeves for the state of Oklahoma is the underenforcement of muffler and noise regulations. Every body and his coon dog is running way louder than stock.

I'm running an oversized commercial muffler on my Dodge/Cummins and 4 inch pipe from the turbo back. It is very free flowing and sounds different but not much louder than OEM. Half the 4 banger rice burners are way louder.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]