Hauling safety

[patrick_g]

During the compression stroke the engine cooling system removes more heat than is generated by the compression stroke. That should mean there could be a retarding force?

Thermodynamics were just not my strong point; in other words I didn't have a clue!

Maybe there are some magnets involved??

Egon, I understand exactly to what you refer and applaud your lack of use of such terms as adiabatic in your description BUT the effect to which you refer is orders of magnitude less than the effect observed and reported and must be ruled out as THE main contributer.

Pat

 

[Defective]

Egon, I understand exactly to what you refer and applaud your lack of use of such terms as adiabatic in your description BUT the effect to which you refer is orders of magnitude less than the effect observed and reported and must be ruled out as THE main contributer.

Pat

So...

You mean we haven't solved Global Warming?

 

[Egon]

Pat, other than transmission somewhere in the realm of thermodynamics should lie the answer.

What's this "Adiabatic" thing?

I can recall seeing signs telling truckers to gear down when steep downward sloped hill was coming up so diesel engines must have some braking ability.

The other thing that perplexes me on a gas engine is that if the air supply is cut off there should be less fluid to compress and hence less braking power??

Defective: After this past winter I would surely appreciate a little warming!

 

[bjcsc]

What's this "Adiabatic" thing?
/

It actually means the opposite of what you were implying. An adiabatic process would occur if the temperature inside the cylinder changed as the compression changed and had no effect on the temperature of the block, i.e. net heat/loss gain = 0.

The other thing that perplexes me on a gas engine is that if the air supply is cut off there should be less fluid to compress and hence less braking power??/

That's interesting as what you suggest is that it would draw a vacuum on the intake stroke, and that coupled with the 50% lower compression ratio would yield significantly less resistance. Sounds plausible...Pat?

 

[patrick_g]

It actually means the opposite of what you were implying. An adiabatic process would occur if the temperature inside the cylinder changed as the compression changed and had no effect on the temperature of the block, i.e. net heat/loss gain = 0.



That's interesting as what you suggest is that it would draw a vacuum on the intake stroke, and that coupled with the 50% lower compression ratio would yield significantly less resistance. Sounds plausible...Pat?

I don't know, I seem to be running out of humorous material. I'm happy that at least one person got the adiabatic thing. It was yet another way of saying, "I don't think so, Tim!"

Egon, since engine braking on a gas engine is not produced by compressing (in the cylinders) the air entering through the intake, changing the quantity of it is not going to produce a big change in retarding force. The retarding force is the work the engine does on the intake stroke pulling air through a restriction (mostly closed throttle.)

The air compressed in the cylinders gives back most of the energy used to compress it. Might as well be springs instead of pistons in cylinders. heat loss in the compressed (AND HEATED) air is not a big factor as I said before. The heat is not that intense and the cylinders are not that cold.

Many if not most large trucks (signs referring to trucks ordinarily do not apply to pickups) are equipped with Jake brakes or equivalent. These brakes do very little in the higher gears compared to lower gears, hence a good reason to suggest trucks use lower gears. A diesel engine in a truck with no exhaust brake and no variable valve timing offers less engine braking than a comparable gasser.

Although retarding force may well increase by an order of magnitude or more by sufficiently down shifting a diesel truck and forcing the engine into significantly higher RPM, the retarding force is provided by friction of moving engine components and is still so weak as to not be useful in most heavy load or downhill situations. What is needed is an exhaust brake or variable valve timing (Jake brake .)

Once while descending from about 11,000 ft on a steep Forest Service road driging a nearly new Dodge-Cumins 1 ton with automatic, I was using 1st and 2nd gear only and was trying to be easy on the brakes, using them only intermittently to try to hold RPM down safely below redline trying to avoid brake fade. It was quite a descent. By the time I got to the bottom of the grade where there was a stop sign I didn't have enough brakes left to actually stop the vehicle short of the cross road. I will always be glad that the Ranger in the Gov truck ahead of me finally woke up and realized that the stop sign he had just been sitting at wasn't going to turn green and he drove on. I nearly hit him. After the brakes cooled they worked again, just fine.

I claim that there was no strategy that would have worked better as I was equipped and that the brakes on the nearly new truck were in fine shape but the task was just too demanding for them. Immediately after returning home from this trip I installed an exhaust brake. I have been down even steeper grades since with good control. I have taken my foot off the accelerator and coasted down a pass on the freeway in California at speeds exceeding 85MPH and been passed by a significant number of other drivers while doing it. With the exhaust brake I have gone down it in 4th and 3rd with neither a perfect match but both were OK. In 4th I exceeded the speed limit (70) just a little (not using friction brakes) and in 3rd we only went 50-55, a tad slow given everyone else's speed.

It may be hard for a flat lander to appreciate the dynamics of a heavily loaded diesel in steep grade and or heavy load situations. This is true whether towing or hauling. Towing can involve trailer brakes which will help if used judiciously and the grades are not too steep too long but hauling puts the whole problem on the truck brakes which in many instances are inadequate to maintain a safe profile irrespective of the gear used or the brake application strategy. In these instances you need more and that is where exhaust brakes are worth their weight in gold (oil?)

Pat

 

[Egon]

Lets go back to yesterday. The engine braking was still perplexing so I was off for a trip down the Google Enquiry road. That proved a dead end but I did encounter the Adiabatic term and some diagrams with strange looking equations attached to them. The diagram was neat and concise so it just may been an idealized version of what really happened in the engine!

Then it was on to home projects for the day. A P/U of slabed wood was cut to firewood length and another load was acquired. A few other necessary home chores were preformed and my aged body became tired and dehydrated. Hydration was imperative but home stores had been depleted. Upon arriving at the proper store it became obvious that it was a federal holiday and home sourced tap water would be the fluid of necessity.

Now on the trip home which is down a quite steep road on which my coasting bicycle can hit 80 KPH with ease it became obvious an empirical diesel engine braking test be preformed. This test indicated there were definite deceleration force's present.

More thought over the evening and thinking about the many tugs and the force required when pulling on the starting cord on my gasoline lawn mower engine with the throttle fully open and a screw driver holding open the choke started to make me suspicious of some of our previous statements.

Then it occurred to me the JAKE system changed the valve timing and my troubled thoughts turned to the camshaft timing and engine valve sizes.

My final conclusion is that the braking effect comes from differences in valve sizing, differences in valve opening and dwell characteristics.

Today it's armchair and rest time. There are limits the old system must adhere too!

 

[patrick_g]

Egon
40 years behind the times

40 might be too conservative and be an underestimate.

Pat