Physics of cold starts - Page 2 - PeachParts Mercedes-Benz Forum

kerry · #16 12-14-2008, 02:01 PM

Quote:

Originally Posted by Chad300tdt

According to the FSM charts and text, it looks like 1978 & 79 models glow 90 + 20 and 50 + 10 for 1980.

The charts show that the glow plug temps max out at 50 seconds, but can have extended glow until up to 80 seconds.

What do the +'s mean?

dkveuro · #17 12-14-2008, 02:05 PM

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

All the VW and other European diesel engines...unless faulty or just plain worn out....start immediately. Some seem to start so quickly it seems they want to start as soon as the key is turned on.

All these are pumping at better than 400 psi.

Another thing that reduces cranking speed apart from draining the battery is oil pressure. Generally oil pressure picks up after about 2 to 5 seconds after cranking. The added drag of the oil pump can slow cranking speed to a 'no start' condition, so cranking continually will increase the load on the starter.

A 1984 300D I built a while ago had 510psi to 520 plus psi pressure after about 8 strokes. It would start at any temperature instantly.

All the pre 1984 Mercedes diesels had no provision for advancing the injection timing for cold starting unlike the Rabbit with the rotary pump. Using this feature helped a lot.

Chad300tdt · #18 12-14-2008, 02:08 PM

Quote:

Originally Posted by kerry

What do the +'s mean?

I was assuming the + " " was the time for ready to start condition on top of the extended glow until the relay timed out.

compu_85 · #19 12-14-2008, 02:29 PM

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

PanzerSD · #20 12-14-2008, 02:36 PM

Quote:

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 · #21 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 · #22 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 · #23 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 · #24 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 ---

JimSmith · #25 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

Chad300tdt · #26 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 · #27 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 · #28 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.

JimSmith · #29 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 · #30 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.