Engine braking in cold temps drops engine temp?

I might have been wrong about the effect of rolling resistance being too small to worry about. I found a reference that gave a typical coefficient of rolling resistance of 0.015. Given that we can estimate rolling resistance losses.

Given:

coefficient of rolling resistance = 0.015
truck mass = 2000 kg

The force produced by rolling resistance is:

F = coefficient-of-rolling resistance * normal-force = 0.015 * 2000 kg * 9.8 m/s^2 = 294 N

This is more than the aerodynamic losses! Can that be true? I've long assumed that at hiway speeds air resistance rules. Maybe I'm too used to thinking about this in terms of bicycles.

Anyway, the adjusted engine braking heat generation is then:

P = force * velocity = (980 N - 190 N - 294 N) * 27 m/s = 13,392 W ~= 13 kW

This is approximately the same as the heat generated by an idling engine.

 

I was a bit short in my comments about this in my "edit", but I'll be honest and say I don't think I fully understand the thermodynamics of engine braking, i.e. where exactly the energy is going. Throttle is closed, and the atmosphere pushes air past the closed throttle, resulting in a large pressure drop (I've seen ~12 in Hg on aviation engines at idle, I'm guessing it's less while running 4,000 rpm..). The largest decelerating force (I think) comes from the cylinders trying their damnedest to expand against this vacuum. Then you have the compression and expansion phases followed by the exhaust phase.. Other than film friction with the lubricated cylinder walls, I'm struggling to see where heat would be generated or transferred to the motor. Am I missing something..? it's obviously gotta go somewhere.

Also, my analysis was completely wrong because I assumed air was entering cylinders at 1 atm (open throttle). I'm guessing you can further divide my estimate by a factor of 3 to account for the lowered pressures behind the closed throttle.

 

There's also friction caused by viscous forces of the engine parts moving through the oil and parasitic loses from the fan, alternator, etc. I can't think any others right now. I also don't understand any of these losses in detail which is why I think it is better to just use the law of conservation of energy and then assume the lost energy goes into waste heat inside the engine.

 

@arthur106, I just read your edits and you brought up a good point. Part of engine braking losses might go out the tail pipe instead of into the engine coolant. I'm not sure how to estimate that without knowing the exhaust gas temperature and rate. Wouldn't the same be true of cooling due to the Atkinson cycle?

One other thought is that it's not clear to me that cooling due to forced convection can be ignored event with the thermostat closed. I made the assumption that it is the main contributor to cooling and did't even think about the Atkinson cycle. Cooling due to the Atkinson cycle is interesting though.

 

So over the weekend I went to the local ski resort and paid attention to my temps, rpms etc coming down the mountain.

Left the parking lot on a cold engine (no idle warmup) with heater set to high on defrost. Ambient temps were 28-32F with snow falling. It's roughly 7 miles and a 2000' drop to the bottom. Right about the time I reached the bottom the temp gauge was in the middle. Average rpm was around 2300 and minimal throttle was used.

 

Your front Disc brakes are sucking up all your engine heat, switch to drums.

 

It’s not clear what we can conclude from this. Are you saying you temp dropped or are you saying it stayed the same? Also were you using your brakes or using engine braking?

 

Sorry, I typed that out quick.

I'm saying my engine gained temperature going down the hill engine braking. It was a completely cold engine at start up, followed immediately by heading downhill. By the bottom of the hill was up to temp. Pretty much all engine braking, with a few brake applications for slick corners with virtually no gas used in the 7 miles.

 

You win. Well done sir!

 

I will assure the OP that the thermostat is not stuck open because if so, the pcm will throw P0128 code (thermostat rationality). Every time you start it there is a timer for how long it takes to get to 178 degrees coolant temp, then full temp. Even if the thermostat was out of calibration, let's say fully open by 180-185 degrees, it will turn on the MIL in cold weather.