Oil & Fuel Diesel for Dummies

[N80]

I'm a new tractor owner and have never owned a diesel anything. And while I don't have any intention of doing any engine work, I would like to understand some basic principles of how the engine works. As background, I have a fair understanding of gasoline engines and used to do some basic engine stuff myself like installing aftermarket headers and carbs and stuff like that.

I understand that a diesel creates combustion from the heat of compression. I understand that fuel is injected at the top of the compression stroke by an injector and that compression ratios are much higher than those in gasoline engines. I guess the main thing that I don't understand is the fuel injector system. In a gas engine the fuel is injected during an intake stroke along with air. But if a diesel injects the fuel at the top of the compression stroke it must have to inject the fuel against tremendous pressure. What generates the pressure within the fuel system to do that? Is it some sort of pump? What drives the pump, is it electric or mechanically linked to the engine? How is the timing of the fuel injector controlled, on a gasoline engine the valve timing coordinates intake of gas and air? Also where is the fuel injector located and what does it look like? (I have a Kubota L4400, but I'm not really looking for exact location, just generally where are they located.) There seems to be a virtual octopus of high pressure lines and hoses on the right side on my engine.

Also, the engine obviously still has valves for air intake and exhaust. In these small tractors with straight 3 or 4 cylinders, is it a single overhead cam that drives the valves? Is the cam belt or chain driven?

Finally, I understand that a glow plug is used for cold starts. Is there a glow plug in each cylinder?

Thanks for any pointers you smart folks can provide.

 

[Highbeam]

I'll try.

Very high pressure for fuel injection is created within the fuel injection pump. The fuel injection pump is mounted to the engine and is gear driven to inject fuel at the exact right time. The fuel injector is oriented and looks pretty much like a spark plug. One for each cylinder and fuel is injected downstream of the intake valve either directly or indirectly into the combustion chamber. Each injector has a metal high pressure line to it and usually a low pressure line away from it. The cam is usually in the block with pushrods to operate overhead valves. The cam is driven by the crank by a gear and then the cam gear in turn drives the injection pump. This way the timing of valves and fuel injection jives with piston position. The above setup is the most common and oldest way of doing it.

Then realize that the diesel does not use a throttle plate. When you ask for more rpms you are only adding more fuel. Each time the piston sucks air it is getting a wide open throttle gulp of air regardless. The way to shut a diesel off is to stop the fuel flow.

There is a glow plug for each cylinder of most diesels. Some diesels use intake air heaters which is a heated metal grid, and some use nothing at all like my old 1972 international diesel which started fine down to 35 with no trouble. The glow plugs also look like spark plugs but have electrical connections instead of fuel lines.

 

[N80]

Thanks, that helps a lot. But, I asked one question wrong. I know where the actual injectors are located and what they look like, what I guess I was trying to ask is where is the injection pump and what does it look like?

 

[JerryG]

<font color="blue"> it must have to inject the fuel against tremendous pressure </font>
That is correct.
<font color="blue"> What drives the pump, is it electric or mechanically linked to the engine? How is the timing of the fuel injector controlled </font>
The pump is gear driven off the front of the engine. Just like the cam works by being set on certain timing marks along with the rest of the gears in the front timing cover.

<font color="blue"> where is the fuel injector located and what does it look like? </font>
It sound like you know some about gas engines. Look on the diesel engine where the spark plugs would be on a gas engine. They look to be all metal, but instead of wires going to them, a small steel line goes from one injector to the next.

<font color="blue"> In these small tractors with straight 3 or 4 cylinders, is it a single overhead cam that drives the valves? Is the cam belt or chain driven?</font>
All of the small diesel tractor that are spoken about on TBN are cam in block as far as I know. Everything is gear drive, no chains or belts.

As for the glow plugs. Most modern small diesels that have glow plugs have one for each cylinder, but there are some that use only one. Some use other types of heating systems.

 

[Highbeam]

There are a couple of different styles of injection pumps be it inline or rotary but they look pretty much the same. To find your pump, follow the metal injection lines from the injectors to what looks like a distributor cap. Where the metal lines all stop together is the pump. It will be round and about the size of an alternator. The low pressure rubber lines will also lead from the fuel tank and filter to the inejction pump. It will be near the front of the engine.

 

[daTeacha]

Fuel injected gasoline engines in newer cars have a computer controling the timing of the injection of the charge as well as the amount of fuel injected. On newer tractors that meet tier II emission regulations, is this also the case, or do they retain the basic mechanical controls described above?

All the fuel injection systems operate under very high pressure -- thousands of psi. This is needed not only to overcome cylinder pressure, but also to atomize the fuel for efficient combustion.

 

[HGM]

Just to add a bit to the pressure thing.....I'm not sure of your model, the other guys seem to have covered it pretty well. The pressures your dealing with could be up to 30,000PSI. I dont think yours would be that high though because of the injector pump design, but even 20,000 will cut you. My point is, that if you are doing any work on it, and think it may have a fuel leak, dont feel for it with your hands.The fuel can enter your skin and poison you. We used to recomend using a peice of thick paper to trace the lines looking for a leak.

 

[tlbuser]

</font><font color="blue" class="small">(
The way to shut a diesel off is to stop the fuel flow.
)</font>

Or cut off the air flow. Both of which really applied to older engines. Newer stuff, being all electronically controlled, cutting off the juice will stop the process also.

 

[robison]

HGM . . . Diesel injection pressures are two to three thousand PSI, not twenty to thirty thousand.

Still enough to cut into you but not quite as high as you said.

Another post above suggested gas used the same injection pressures . . older gas engines inject fuel into the intake manifold, and they have fuel pressures in the 40-90 psi range.

There is a brand new technology from Bosch - gasoline direct injection - that injects gas right into the combustion chamber like a diesel. It will use pressures at 750-1500PSI. This technology promises 8-10% increases in power and economy. This system is not presently on sale in the US but it's coming.

Our company is authorized Bosch automotive system service for Western MA.

 

[HGM]

Yes, on this system I believe your right. I thought about that after I posted. The newer DI and common rail systems are the ones I was commenting on. They are up to 30,000PSI out of the nozzle. Sorry for the confusion, I think my point on safty was clear though. /forums/images/graemlins/grin.gif