Physics of cold starts - PeachParts Mercedes-Benz Forum

Brian Carlton · #1 12-14-2008, 02:38 PM

Quote:

Originally Posted by dkveuro

Not to burst anyone's bubble....but compression efficiency is key to starting a diesel.

Since raising the compression of a 20 year old engine is effectively impossible for nearly everyone, the discussion must turn to more mundane approaches such as glow time, battery voltage, oil viscosity, starter condition...........etc.

I will state, however, that the new valves on the SD did absolutely raise the compression for start and I'd seriously advise anyone to consider it.

The SD would start to struggle at 15°F. with 10-15 second crank times.

Now, it starts in one second at 0°F.

Hatterasguy · #2 12-14-2008, 04:05 PM

I have noticed this with all diesels, when its real cold out you get one shot. If you don't start it on the first try it needs a lot more cranking, and even a very good battery will only supply so much power when its at or below zero.

81300sd · #3 12-14-2008, 04:49 PM

Never had an issue with my SD, started this morning @ 2 degrees with only about an 8 second glow. Probably about 8 second crank till it was on its own.

My Ford, however if I let off the starter too soon or don't glow it long enough does require more glow time and is a bit more difficult the second time. I have manual glow plugs, so I just hit the glow for another 5 seconds or so and crank till it fires.

71inka02 · #4 12-14-2008, 06:47 PM

So a question to clarify for me --- Does, cycling the 'glow' process a couple of times help on cold morning starts?

When I say cycling - I mean turn key to glow - glow goes out, turn key off then turn back to glow ---

Chad300tdt · #5 12-14-2008, 07:12 PM

Quote:

Originally Posted by 71inka02

So a question to clarify for me --- Does, cycling the 'glow' process a couple of times help on cold morning starts?

When I say cycling - I mean turn key to glow - glow goes out, turn key off then turn back to glow ---

It makes a difference on both of my 123's.

nickofoxford · #6 12-14-2008, 07:40 PM

Quote:

Originally Posted by 71inka02

So a question to clarify for me --- Does, cycling the 'glow' process a couple of times help on cold morning starts?

When I say cycling - I mean turn key to glow - glow goes out, turn key off then turn back to glow ---

Not at all, I used to think the same thing until I learned how the glow system works. The Glow plug light is only a recommendation to how long you should glow before you start. In reality the glow plugs are still being powered after the light goes out for a certain amount of time, my 240D for example is around 30 seconds. So cycling is only extra wear on your ignition switch, we all learn this trick sometime

. I really want to hook up an extra indicator light to tell me when my glow plugs really shut off. The relay is a good way to tell, if your driveway is as quiet as a mouse in the morning.

Chad300tdt · #7 12-14-2008, 07:58 PM

Quote:

Originally Posted by nickofoxford

Not at all, I used to think the same thing until I learned how the glow system works. The Glow plug light is only a recommendation to how long you should glow before you start. In reality the glow plugs are still being powered after the light goes out for a certain amount of time, my 240D for example is around 30 seconds. So cycling is only extra wear on your ignition switch, we all learn this trick sometime

. I really want to hook up an extra indicator light to tell me when my glow plugs really shut off. The relay is a good way to tell, if your driveway is as quiet as a mouse in the morning.

What he was talking about is letting the glow plug relay click off at around the 30 seconds you refer to not waiting for the ready to start light to go off. (at least that's what I interpreted) A second complete glow cycle DOES benefit a start in very cold temps. I do it on both my w123's and it makes for better starts.

You don't need a light to indicate when the relay clicks off, just listen for it. It's a fairly loud click at least on the newer glow relay systems.

nickofoxford · #8 12-14-2008, 08:37 PM

Quote:

Originally Posted by Chad300tdt

What he was talking about is letting the glow plug relay click off at around the 30 seconds you refer to not waiting for the ready to start light to go off. (at least that's what I interpreted) A second complete glow cycle DOES benefit a start in very cold temps. I do it on both my w123's and it makes for better starts.

You don't need a light to indicate when the relay clicks off, just listen for it. It's a fairly loud click at least on the newer glow relay systems.

OHH gotcha, I always manage to look like an idiot on here somehow.. it definitely will help as you said, Is there a way to just modify the set glow cycle or does that just require a whole new relay? Where I live the street noise in the mornings makes it hard to listen for the relay, so a light would help out. It's more of a hit or miss situation, depending if my neighbor takes his 240D by my house at the right time.

strelnik · #9 12-22-2008, 03:49 AM

Quote:

Originally Posted by nickofoxford

I really want to hook up an extra indicator light to tell me when my glow plugs really shut off. The relay is a good way to tell, if your driveway is as quiet as a mouse in the morning.

This is one of the really good features of the old diesels. You can glow up to 2 minutes with the old Bosch " candle wire" injectors or the Beru "bird cage" injectors.

It went below zero last night so I decided to plug in the block heater. I'll also give the battery a little zip this morning before I start the 240D. This year in the summer, the 190D (and the "hybrid" 1940 170 I'm making into a diesel) will get block heaters, assuming I can find something that fits, which I believe I can.

S

JimSmith · #10 12-14-2008, 08:05 PM

Quote:

Originally Posted by 71inka02

So a question to clarify for me --- Does, cycling the 'glow' process a couple of times help on cold morning starts?

When I say cycling - I mean turn key to glow - glow goes out, turn key off then turn back to glow ---

I think that is likely a good practice, if your battery is in great shape. If the battery is weak, what you gain in prechamber heating, and glow plug filament temperature, is quickly overwhelmed if the starter turns a bit slowly because the voltage is down slightly.

The successful starting sequence in an older Diesel is a tightly choreographed set of events that must be synergistic - meaning together they accomplish something that is greater than could be anticipated by the evaluation of the effects of each subsystem element evaluated in isolation. This also makes it confusing to diagnose poor starting performance - the systems and subsystems examined in isolation don't seem so bad.

Jim

Hatterasguy · #11 12-14-2008, 08:09 PM

If you can plugging the car in does wonders. Thats certianly the way to go if its at all possible.

nickofoxford · #12 12-14-2008, 08:42 PM

One more thing...

Quote:

Originally Posted by Hatterasguy

If you can plugging the car in does wonders. Thats certianly the way to go if its at all possible.

Hatterasguy,

AMEN.

Sincerely,
Nick

JimSmith · #13 12-14-2008, 07:11 PM

I believe one of the problems is, after the first attempt that results in a "no start" the glow plugs spend a bunch of time evaporating liquid Diesel and until the liquid is off the exposed glow plug material, the temperature of the glow plug is limited to the temperature at which Diesel fuel will evaporate, at the pressure in the cylinder. This leads to, once you have failed to ignite, a compression stroke that raises the critical temperature for the Diesel to evaporate, followed by a cooling during the exhaust stroke, and another wetting, etc. until what heat of compression that is created in each compression stroke, is absorbed into the adjacent metal, allowing the internal temperatures to begin to rise.

For a reliable start, yes, you need some minimal compression. On later Diesels with turbo's the starting compression ratio is significantly lower than, say, a 616 style 240D. So, compression is not the whole story.

Later glow plugs reach significantly higher temperatures and are much less susceptible to being wetted. In addition, the injectors of later Diesels inject a short, small duration fine mist followed by at least one and in some of the piezoelectric injectors, several more squirts of smaller volumes of Diesel. This, when the fuel is particularly thickened and the spray pattern compromised, leads to an easier ignition of the first charge, which then increases the combustion chamber bulk temperature significantly and then results in the second and later injection spurts being injected into a flame front.

The key would be to fit some of these new ceramic glow plugs that reach even higher temperatures on the older Diesels. It would also be great if MB or the Indian guys making the Ghurka models would supply a replacement injector that is capable of the pre-bulk injection spurt like the 606 series engines have. That is purely a pintel and nozzle configuration change and it would help with efficiency and smoking as well.

Preheating the air might also help, but in dead cold with an old engine the combination of lower basic starting compression due to age, likely less than optimum valve settings, a less than optimum battery and starter condition, likely un-winterized fuel, and borderline injectors in an engine filled with a good Diesel engine oil that is not synthetic (meaning it is likely a 15W-40 or higher viscosity), the chances of good starting are slim to none. The starting event for a 616/617 must be a coordinated attempt to cheat Mother Nature. Any one of the subsystems involved that is not performing up to snuff will stress the others. If any of them can't handle the extra load, Mother Nature snuffs out the attempt.

So, you have to make sure all the systems and subsystems involved in the attempt to start are in good working order. And, there are some advances that make the starting events less susceptible to failure - such as the newer injectors with their pulsed injection sequences, hotter glow plugs and synthetic engine oils (such as Delvac 1 by Mobil that advertises a 5W-40 viscosity characteristic, but flows at sub zero temperatures with much less drag on the starter than any Dino products for Diesels, and there are several similar synthetic products from other suppliers, so Delvac 1 is not the only choice).

Jim