Physics of cold starts

[kerry]

I've been starting 617's in cold weather for about 7 years now. I (and others on this forum) have noticed that in very cold conditions, the first starting attempt is always the best chance to get the engine going. Any subsequent attempts to start the engine, even if glowed as long as the initial attempt, are more difficult. Why is this the case? It almost seems as if subsequent attempts after the first are only successful once the heat from turning the engine over is enough to ignite the fuel. In other words, the glow plugs seem less effective in the later start attempts?
These thoughts are spurred by the fact that I started the 116 at 5 degrees this morning without pluggin it in after replacing a glow plug on Friday just to see if the glow system was working correctly again. I glowed for 75 seconds and engine fired after about 3 or 4 revolutions but I foolishly let off the starter before it was running steadily enough and it stopped. The next attempt, even with a 75 second glow and the engine previously running momentarily, took a good 30 seconds of cranking before a single cylinder fired and then another 15 seconds of cranking before enough cylinders were firing to keep it running.

Any good explanations out there?

[Brian Carlton]

It's the problem of continuous liquid fuel accumulating in the cylinders, thereby requiring that much more heat to fire.

Your best hope to start is in the initial 1-5 seconds. The prospects deteriorate based upon time.......not the number of attempts.

The deterioration with time is offset by the increase in the temperature of the cylinder due to repeated compression strokes.

Additionally, the deterioration with time is caused by decreasing battery voltage, thereby resulting in reduced crank speed and subsequently lower compression.

It's an interesting dynamic.

[Brian Carlton]

Originally Posted by kerry

I glowed for 75 seconds..........

How were you able to accomplish this and overcome the relay timeout?

[kerry]

Originally Posted by Brian Carlton

How were you able to accomplish this and overcome the relay timeout?

On the 116, that's the relay's timing. It stays on for 75 seconds. On my 85 TD it's only 28 seconds. The 116 has a physically different relay than the 85 TD. Don't know if this is a factor in the timing.

[kerry]

Isn't that liquid expelled each exhaust stroke, showing up as that grey unburned fuel exhaust? Is that liquid fuel just cooling down the cylinder causing the problem? If it is, would injection line heaters overcome the 'one try' problem?

[Brian Carlton]

Originally Posted by kerry

Isn't that liquid expelled each exhaust stroke, showing up as that grey unburned fuel exhaust? Is that liquid fuel just cooling down the cylinder causing the problem? If it is, would injection line heaters overcome the 'one try' problem?

I believe that the liquid fuel must be sitting on the piston face and on the top of the cylinder to a significant degree. If most of it were expelled on the exhaust stroke, the issue wouldn't exist.

I'm sure injection line heaters would help significantly. The cylinder would much prefer fuel at 180° versus 0°.

[PanzerSD]

Originally Posted by kerry

Isn't that liquid expelled each exhaust stroke, showing up as that grey unburned fuel exhaust? Is that liquid fuel just cooling down the cylinder causing the problem? If it is, would injection line heaters overcome the 'one try' problem?

With that I can imagine, a cold engine with it's cylinders ICE cold, recieving cold diesel. The volume of fuel in your tank is enough that with it's total mass and confined space, it will not gel. wihen you inject that diesel fuel now aspirated by the injector coating the inside of the cylinder, and piston surface instantly gelling on contact, much like the concept of making a curling rink. (yeah, I'm Canadian)

[Brian Carlton]

Originally Posted by kerry

On the 116, that's the relay's timing. It stays on for 75 seconds. On my 85 TD it's only 28 seconds. The 116 has a physically different relay than the 85 TD. Don't know if this is a factor in the timing.

Interesting..........does it use different glow plugs?

[kerry]

Originally Posted by Brian Carlton

Interesting..........does it use different glow plugs?

No, unless I'm buying the wrong plugs. They're regular pencil plugs.
The relay is bigger and shaped different.

[Brian Carlton]

Originally Posted by kerry

No, unless I'm buying the wrong plugs. They're regular pencil plugs.
The relay is bigger and shaped different.

I'd be curious if a 30 second time gave the same results as a 75 second time.

Since it's the same engine, one might conclude that M/B learned that they didn't need the longer time and it served no benefit. The plugs can't heat the cylinder or the prechamber in any fashion..........only the air in the immediate vicinity of the plug.

[Chad300tdt]

How long was the glow light on for? How long after the glow light went out until the relay clicked off?

From what I understand from the FSM, the Ready-to-start condition (glow light off) is variable depending on ambient temps and the time frame between the ready-to-start condition until the relay clicks off is fixed.

Maybe the 5˚ temp caused a longer time for the ready-to-start condition?

[compu_85]

Note that the later "afterglow" relays don't have the "glow + timeout" feature anymore, they will run for up to 30 sec, including the time the indicator light is on.

-Jason

[Jeremy5848]

Plugging the battery into a trickle charger overnight not only keeps it topped off but also keeps it a little warmer than it otherwise would be, due to the waste heat generated in the battery as part of the charging process. Cold batteries have less energy than warm batteries.

If you can't plug in (battery trickle charger and/or block heater), consider keeping an extra battery in the house, connected to a trickle charger. When it's time to start, carry or wheel the battery out and jump the car's battery, then easily start the engine.

For a DIY on installing afterglow in your W124 car, simply and inexpensively, see this thread. Something similar could be done for the W123 and others.

Jeremy

[Chad300tdt]

Originally Posted by Jeremy5848

Plugging the battery into a trickle charger overnight not only keeps it topped off but also keeps it a little warmer than it otherwise would be, due to the waste heat generated in the battery as part of the charging process. Cold batteries have less energy than warm batteries.

If you can't plug in (battery trickle charger and/or block heater), consider keeping an extra battery in the house, connected to a trickle charger. When it's time to start, carry or wheel the battery out and jump the car's battery, then easily start the engine.

For a DIY on installing afterglow in your W124 car, simply and inexpensively, see this thread. Something similar could be done for the W123 and others.

Jeremy

Nice write up Jeremy. I especially like your afterglow indicator light.

[kerry]

More grist for the mill:
17 below zero F here this morning. Both cars plugged in . My wife went out to start the 116. It fired on the first compression stroke but my wife let off the starter before it was fully running and it stopped. She tried repeatedly to start it. No go.
This further complicates the 'you only get one chance to start it' issue. Since the engine was relatively warm (I opened the hood to confirm), I would have expected it to start on the second try or third try. Could it be that the fuel in the immediate vicinity of the engine is warm from the heater but the initial start attempt circulates that warm fuel away from the engine and all later attempts are using cold fuel and that is the major factor in the 'one chance'?

The TD started on the first compression stroke. After I confirmed we had one running vehicle I went to the 116 to give it another try. This was about 10 minutes after my wife's failure. No firing at all after a 30 second glow and I had to crank it for about 30 seconds before a single cylinder started firing and another 10 seconds before enough cylinders were firing for it to run on its own. This is with a warm block.