Why do some diesels require glow plugs and others don't?

[Gary Fowler]

My old Yanmar 4220 doesnt have glow plugs and I have looked for the precombustion fire bowl or whatever they call it (supposed to have some kind of little fire bowl that if filled with diesel from the return line and has a hot wire that sets it on fire so the fire is sucked into the combustion chamber) I dont see that or any other starting aid and if it is below 50 degrees it is very hard to start. If it is in the 40's I have to pull the air filter and using a hot air gun or hair dryer to inject warm air into the cylinders to fire it up. The 70's model Ford 9000 I had didnt have glow plugs but would start without aid on most mornings. If needed it had a built in ether system. If I remember it has a 301cubic inch engine. I dont have a clue as to why some engines have them and others dont but as one poster said, at some point all diesel engines will need glow plugs. Heck I had trouble with a gasoline engine in my old Cadillac with fuel injection firing at -50 C. It had to turn over several times to "warm up" before it started and even then it ran rough for a few seconds.

 

[ModMech]

The need for Pre-Heating (Glow Plug, Grid Heat or Flame Start) is dependant on may factors, some of which include the following:

- Intended market (operating conditions)
- Compression Ratio
- Pre-combustion or Direct Injection
- Injection pressure
- Injection timing
- Bore/Stroke ratio

Almost ALL pre-cup engines need GPs, this is because the fuel is injected into a cavity in the head (swirl chamber). They are very efficient and produce low soot but high NOx due to the high CR of 21:1 and greater. The heat of compression has to migrate into the chamber and the head acts like a heat-sink on a cold engine, therefore GPs are needed with pre-cup engines.

With direct injection, the other factors become more significant players than with the pre-cup engines. Higher CR designs may not need preheating, if the injeection pressure and nozzle spray is adequate, especially if they are not designed for cold weather use.

Preheating or not, is not as much of an ENGINE design feature as it is a product design feature. Many engines are built w/o preheaters, but they are added by OEMs to ensure acceptable cold-startability of the finnished product.

Most FARM tractors do not need to be used often in cold weather, therefore, they are less likely to have preheaters. UTs and CUTs ARE commonly used in cold weather and it is very common for them to have/need preheating.

Also, many small diesels are pre-cup designs. They are small, light, quiet, fuel efficient and relatively clean. Add to that the fact that they are excellant cold starters with their GP systems and you can understand why they are so common in CUT/SCUT models.

 

[Chilly807]

Glad I read JohnG's post before I wasted an hour typing that answer. He hit the nails pretty much on the heads. Good post.

Sean

 

[whistlepig]

I guess another way to ask this question might be to ask what are the advantages to designing an engine which needs glow plugs to start vs one like mine which doesn't need them. It seems like most of the engines I read about which have glow plugs are newer, smaller machines. Mine on the other hand is 30 years old and 50 HP PTO.

Could it have something to do with the size of the engine or perhaps an efficiency/emmissions thing???

I had the 1964 MF135 diesel for 25 years. If it had glow plugs or any other type of cold weather starting assist I wasn't aware of it because it always started in cold weather. I kind of think something got lost in cold weather starting technology between my old 1964 Massey and my 2006 Kubota.

 

[dcyrilc]

I kind of think something got lost in cold weather starting technology between my old 1964 Massey and my 2006 Kubota.

That's very close to my thoughts and why I decided to ask the question.

 

[ModMech]

Compression ratio and RPM are significant factors in starting DI diesels in the cold, of course not counting the actual temperature. The AD3.152 Perkins has a L O N G stroke and small piston, meaning there is very little area for heat loss during cranking. Coupled with good nozzles and cranking speed, it is easy to see how good cold weather startability was achieved. Of course, one should not discount proper preventative maintenance either

 

[whistlepig]

Would that also apply to a 1950's John Deere model 70? I think it had a 2 cylinder 9" bore and 7" stroke and no glow plugs.

 

[Chilly807]

Sometimes the engine design doesn't lend itself to easy cold starting, but the other benefits outweigh that drawback. Kubota makes an excellent engine, it just needs a little help getting the fire lit in cold weather.

Most of the stuff I work on has direct injection. Although it does start better in cold, cranking speed is always an issue. Slow crank + cold = no start.

Sean

 

[SPYDERLK]

:thumbsup: Good job guys!
larry

 

[Johnbro]

Some Perkins engines have a fuel valve that, on the preheat setting, opens and leaks fuel into the intake manifold. A heated wire in the manifold lights the fuel on fire and then you start the engine which just sucks the fire right in. It takes about 30 seconds for the process.

Another way is to stick the flame from a burning propane torch into the intake and crank it. I've also used burning scraps of newspaper. :laughing:

Or just crank it faster by pushing the car and getting the cranking speed high enough that the cooling effect doesn't count so much.

I had one of those Perkins (4-108) on my last boat (45 foot sailboat). That engine was reluctant to start regardless of the weather. I used to shoot WD40 into the intake and light that with the pre-heater. It would fill up the cabin with smoke and cough about 6 times then finally get running. I'm glad my Kubota is a bit easier!