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Just a curious question. I once read on here that an OM617 should be able to start without engaging glow plugs if the compression is good. Is there any truth to that? Thanks for any and all responses.
Temperature of no glow plug starting.
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'86 W123 200, OM617 non-turbo, bastard 5-speed; '95 W202 C250 Diesel, OM605 non-turbo, 5-spd man
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It does start, but is more likely to drain the battery trying. Glow plugs are not essential to starting a diesel, but it makes starting a quicker and easier affair.
Some modern diesels don't even have glow systems anymore.
Some modern diesels don't even have glow systems anymore.
Check the owners manual, I seem to remember Mercedes saying when you don't need to glow it.
You can likely look up the minimum Ignition temp for properly atomized Diesel Fuel.
I have not read this anywhere but in Trade School I was told that there is a 2 degree temp rise for every psi of Air compressed in the Cylinder. Of course that favors an New Engine over a used one.
After that conduction of the heat of compression into the Engine Components is a big factor as well as the proper atomization of the Diesel Fuel and the timing of the Injection. Not to mention the Starter cranking fast enough which has to do with the Battery, starting system and the Crankcase Oil.
So a lot of stuff in the system has to be working properly for the Engine to start easily without Glow Plugs.
Direct Injection Engines often don't have Glow Plugs but their Compression Ratio is higher and the Injectors Atomize the Fuel better.
Some have other starting aids like Compression releases to aid the Engine in cranking faster at starting or Either Injection.
There is devices that heat the intake manifold Air by way of Electric Heaters or actually burn Fuel in the device.
'86 W123 200, OM617 non-turbo, bastard 5-speed; '95 W202 C250 Diesel, OM605 non-turbo, 5-spd man
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I'm going to disagree on the compression ratio. From my experience, direct injection diesels often have a LOWER compression ratio.
It turns out that for any 4-stroke engine, irrespective of ignition type, the optimum CR is 12. Low octane fuel have prevented spark ignition engines from reaching these high CR, while compression ignition engines required a much higher compression temperature to get heavy oils to ignite. So both types of engine are trying to reach this CR.
The OM617 had a 21.5:1 or 22:1 CR, the OM603 and OM 606 had a 22:1 CR (but don't quote me on this - apparently my sources aren't all that reliable). By comparison, the CDI OM611 has a 19:1 CR. Similarly, Toyota's indirect injection NA diesel 1HZ had a CR of 22.4:1, while the direct injection turbo 1HD-T had a CR of 18.6:1
Turbodiesels may have a higher compression pressure than NA diesels, but that only holds after the turbo is spinning, so it doesn't affect starting. Of course, the higher pressure of modern diesel systems and subsequent finer atomization of the fuel do help the starting at lower CR.
I totally forgot something.
On the Engines with the Pencil type Glow Plugs (you need a 12mm Wrench to remove them); when the Starter is cranking your Glow Plugs are also on.
I think the same is true of the Filiment/Loop style Glow Plugs on the later 1970s Models but I am not sure.
The glow plug relay has its own temperature sender to tell it how long the preglow cycle should be. The preglow does not affect the temperature reading of the sender; only actual cylinder head temperature does this. You can get multiple preglow cycles of the same duration by turning the key on and off. I am running 80/20 used motor oil/gasoline ($0.03 per mile), and I need to preglow twice at least, when cooled overnight. When my 617 is hot, the preglow cycle is very short, less than 1 second, or non-existant. I use a high pressure electric fuel pump to give me 14 psi at the injector pump, so that I get first-turn ignition when the precombustion chambers are hot, and the engine is cold. The OEM lift pump takes many revolutions to build adequate charge pressure. Excessive cranking ruins the battery and starter, as well as the flywheel gear teeth. These are expensive fixes. My 617 has been recently overhauled, but it will NOT fire when cold without extensive cranking unless preglowed and primed. Preglowing also reduces exhaust smoke on startup, since proper combustion requires that the precombustion chambers be very hot. If you want to run diesel (or other stuff) you have to have the patience to follow procedures. Otherwise, stick to gassers.
I was going by what I was taught in Trade School. It had that IDI Injection Diesels could be as low as 17:1 Compression Ratios and that DI Diesels were 20:1+ Compression ratios.
However, back in the 1970s when I went to Trade School few Cars had Diesel Engines. Most IDI Injection Engines were in equipment.
If they were in Vehicles is was things like Tractors and Forklifts.
Most of the Trucks in My area at that time had Detroit Diesels or Cummins Engines in them with a smattering of CAT Engines and even more Rare Mack Diesel Engines.
Engines with Turbo Chargers have been know to have slightly less compression Ratios than the Naturally Aspirated Versions.
With my car parked inside the unattached garage and a 50 degree outside temperature, car wouldn't start the next morning. Found out that I forgot to reattach the plug from relay to glow plugs when troubleshooting the relay. Plugged in, car started right up after normal GP cycle. Never checked my compression, but with 212K runs good, don't suspect compression is abnormal. 1982 240D
This not particular to Mercedes Diesels but generic Diesel information.
"In the true diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa; 120 to 200 psi) in the petrol engine.
This high compression heats the air to 550 °C (1,022 °F). At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber."
That info is buried in the below site:
Diesel engine - Wikipedia, the free encyclopedia
I have yet to find what minimum Compression will yield that temp.
Note; a lot of People believe that when their Cranking the Starter the Glow Plugs are not on. That is not true and there is a good reason to keep the Glow Plugs on during Starter Cranking.
With the Pencil type Glow Plugs the Heat zooms up to 2012 degrees F and levels off at 1832 degrees F. That is a higher temp then the cranking Engines Compression Can produce (especially in a worn Engine) and that heat is localize in the Pre-combustion Chamber so there is a decreased area for the heat to get lost in.
So Mercedes rightly made the choice to divert Electricity to the Glow Plugs to get a Higher Heat during Cranking then to shut the Glow Plugs off in order to gain a bit of Starter Cranking speed that even in a perfect Engine cannot bring get the Pre-combustion Chamber temp as high as if the Glow Plugs were on at the same time.
"In the true diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa; 120 to 200 psi) in the petrol engine.
This high compression heats the air to 550 °C (1,022 °F). At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber."
That info is buried in the below site:
Diesel engine - Wikipedia, the free encyclopedia
I have yet to find what minimum Compression will yield that temp.
Note; a lot of People believe that when their Cranking the Starter the Glow Plugs are not on. That is not true and there is a good reason to keep the Glow Plugs on during Starter Cranking.
With the Pencil type Glow Plugs the Heat zooms up to 2012 degrees F and levels off at 1832 degrees F. That is a higher temp then the cranking Engines Compression Can produce (especially in a worn Engine) and that heat is localize in the Pre-combustion Chamber so there is a decreased area for the heat to get lost in.
So Mercedes rightly made the choice to divert Electricity to the Glow Plugs to get a Higher Heat during Cranking then to shut the Glow Plugs off in order to gain a bit of Starter Cranking speed that even in a perfect Engine cannot bring get the Pre-combustion Chamber temp as high as if the Glow Plugs were on at the same time.
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