davelinde

I've only had my truck a year and I live in a moderate climate. Still, last night when it dropped to 10F I decided it might finally be time to use the block heater. I was pleased to find that the wait-to-start light popped off just as fast as it does in the summer and the truck started quickly and ran quiet immediately. Now coming home from work tonight the wait-to-start took a more typical 20 seconds or so and the truck rattled along for 3 minutes of driving before smoothing out.

Now... just out of curiousity. What is the heater heating and what does it help? I think it's heating the coolant in the block around the cylinders. How does that reduce the time for the light? What else is it doing? I just went and got a timer for it, so I can plug in without heating it for 6 hours. Is this a good idea? Temps here will not stay at 10... more typical is 30's most of the winter.

tool

Your right.

Block heater is just an electrical heat element which heats the coolant and warms the engine. (since the colloant is warm the rest of the engine is warmer too)

Dieseldude4x4

The block heater is nothing more than a mini water heater element that sticks in the side of the block below the turbo in that general area. It is rated at like 650 or 750 watts and heats the water in to coolant system. Through natural circulation in the block, the whole coolant system warms up. I plug mine in when it gets about 20 and just leave it plugged in.

hotmopr

I almost always plug mine in under 40*. I like having defrosters almost immediately.

dalton4

The "wait to start" light has a sensor that activates the light and intake grid heaters when engine temp is around something like 40 degrees F. Using the block heater warms the engine above this temp.

davelinde

ok... I understood that plug was to a resistive heater in the block.
The coolant gets warmer, so the cylinder walls are warmer.
It must be true, 'cause it works... but it strikes me as interesting that it matters in the scheme of things. Does anyone know roughly what the various temps are?
At full temp the coolant is 190ish initially I only know it's heated to less than 140.
Initially, post turbo, the EGT is 190ish too? and with 0 boost is the pre-turbo temp also that low?
Intake air would be about 10 (ambient) and flows over a grid to get to what temp?
After the compression the air is up to what? 1400? more?

Again, it surprises me that the cylinder wall temp matters, but clearly it does, changing things like ease to start, smoke, and noise.

Thanks for info. This is just my indulgence in pure curiosity for the day.

Raspy

Dave,

It is interesting that a small temp difference can make such a big starting difference. But remember the whole game is getting the compression temperature up just high enough to reliably ignite the fuel. This is related to incomming air temp, compression ratio, and inside surface temps of the piston, cylinder head, and cylinder wall, elevation, and of course, time.

If the engine cranks too slow the heat will be lost. If the pressure is too low, a high enough temp will not be reached and this is a tricky one because a higher compression ratio would give a higher temp but would be too high under boost conditions. It seems the compression ratio of about 17 to one is almost as low as can be used reliably in a diesel engine and still get good starting. Also the direct injection (no pre-combustion chambers) and a relatively large combustion chamber means a hotter zone is available for ignition because the heat path is greater for the air to a cold surface. The heat path and exposed surface areas are an important part of the equation. These Cummins start much better than a Mercedes, or VW, or Izuzu, or Fords, or Kubotas. They all seem to have some sort of pre-combustion chamber which means more exposed surface area when starting, and some have much higher compression ratios. I think my little Kubota is 23 to 1 and it is hard to start at 2800 ft. and about 35 degrees unless I leave a light bulb next to it or pour hot water into the hopper .

So, if the manufacturer sets the compression ratio as low as can be used reliably under normal conditions to acheive ignition temperature, we end up out of the envelope at about 0 degrees at sea level. My old twelve valve would not quite start without the grids at 6,500 ft and 9 degrees. My 3rd gen starts without the grids at 4500 ft and 15 to 20 degrees but misses a little at idle. It's funny how a small difference in temp can make all the difference in starting

To bring things back into the envelope we need to raise the air temp slightly by using the grid heater. And remember, once the engine starts it should have no trouble continuing to run because it's turning faster so the heat loss to the head and piston has less time and there is less loss past the rings so pressure should be a bit higher than when cranking. Then it begins to warm up to operating temp and settles into normal operation.

Diesels are so cool.

Wetspirit

RustyJC

Actually, it depends on the surface area to swept volume ratio. As you said, small diesels like your Kubota may use 23:1 compression ratios. Medium size diesels like our Cummins may be around 17:1. The Cooper Bessemer LSVB diesel (15.5" bore x 22" stroke) has an 11:1 compression ratio. The tendency, therefore, is that as the engines get larger (i.e., less surface area per unit of swept volume), compression ratios can be lowered.

Rusty

davelinde

Thanks for the info!

I was googling a little and found this "the difficulty in starting diesel engines is caused by combustion instability at low temperatures. Combustion instability will cause the engine to misfire once before it fires again. This is referred to as 8- stroke-cycle operation. If it misfires twice, it is referred to as l2-stroke- cycle operation, and so on. This pattern was found to be reproducable. The engine may start on a l2-stroke-cycle operation at low temperatures, shift to an 8-stroke-cycle, and finally shifts to the regular 4-stroke-cycle."

I also think I found that the temps at combustion are ~500F+?

It still seems interesting that cylinder wall temps influence the startup. The air rises in temp under compression quickly and I would not expect a lot of heat transfer between the cylinder wall surface and the air in the short time between compression and combustion.
What was this the ratio noted? eg surface area of the cylinder to cylinder volume? I can see an intuitive link there now that I think about it. Not sure I understood what is meant by swept volume?

Thanks again, fun to be curious. Yes diesels are interesting beasts, glad I got mine.

Raspy

davelinde,

The heat transfer from the compressed air to the cylinder wall is an interesting subject. Since the volume increases faster than the inside surface area in a spherical volume, bigger displacement engines and ones with fewer cylinders can be better at starting, especially with direct injection, because the heat path is longer and there's more volume per surface area.

The idea of "time to cool" is also interesting to me. It seems like some time must pass to cool the charge, but it is not much. A study of stirling engines tells a lot about this change in temp idea. They simply give up heat or absorb heat to cause a change in pressure, and thus a driving force similar to, but much weaker than, combustion. Stirling engines can run at quite high RPM. All the while giving up heat to surroundings or absorbing heat from surroundings to make power through the resulting pressure differential. I have one that runs at about 3000 RPM!

Bear with me here. Imagine a room filled with bouncing *****. ***** going in all directions. Bouncing everywhere.
Now imagine the walls made of a soft material that absorbes the impact of each bounce. It would not take long before all the ***** had much less energy. Now imagine the walls hard and vibrating. Each time a ball hits it is struck back and sent flying into the room with high energy. This difference between the two wall types is the difference between hot and cold. It's not so much that it takes time to cool the air as it is that the molecules give up energy to their surrounding. Temperature is just a measure of the molecule's average kinetic energy. Looking at it in this way helps me justify what I see in running engines.

The "time to cool" idea has been a wonder for me for a long time. And the stirling engine is a great teacher.

Wetspirit

cp

Great explanation, Wetspirit.

The fact that air can heat up in the amount of time it takes to be compressed ought to tell us that temperatures can change pretty rapidly (both up and down) inside a cylinder.

greatwhite

Block heaters work for gas engines too. My 383 and my 440 have dual block heaters, one on each side. If I plug in both of them at -30F, within 2 hours, it will be blowing warm air out the defrost vents as soon as it is running. They are 1000 watts each. Same deal as the diesel, you are just that much closer to operating temp when you light it up. It's just easier on everything.

davelinde

Stretching my memory of Thermodynamics....

I had an intuitive feel for how compression raised temp - in fact I thought that the rate of change in pressure was related to the delta Temp?

I was kinda thinking that the cylinder wall interaction with the charge would use conductive heat transfer only (eg no time for convection and not much radiant transfer). I was visualizing a thin boundary layer of air that would actually insulate the cylinder wall from the majority of the charge.

Maybe it doesn't work that way? Has anyone ever seen a thermal map of this?