Diesel Engine


A diesel engine is very efficient for the power that can be developed, the simplicity requiring no spark plug or electrical system to run, and the long life they are known to have in equipment makes them a preferred engine to have in equipment requiring heavy service.



There are very few problems that happen to the diesel engine however they do have some issues regarding clean fuel. A common field call for a diesel engine is "engine will not start" . Most of the time it is fuel related and there are various versions of fuel systems. In most newer equipment the fuel injection pump has an electric operated shutoff with one wire. When current flows to the shutoff coil it opens the valve from the fuel transfer pump. The fuel injection pump is the pump with the metal tubes bolted on that deliver flow to the injectors that inject the diesel fuel into the combustion chamber of each cylinder that particular engine has. The injection pump has one steel tube for each injector and the injection pump shaft is gear driven from the engine timing gears. If the engine you are working on has 4 cylinders it has four steel tubes on the injection pump. Also there is a tube or hose that allows low pressure fuel to enter and one steel or rubber line that allows the case of the injection pump to flow fuel out to the return in the fuel tank.


To identify the injection pump from the transfer fuel pump simply count the lines attached to each pump. The fuel transfer pump has one inlet line and one outlet line and some times one by-pass line. The injection pump has more lines and tubes attached to it. The fuel transfer pump makes diesel fuel flow from the reservoir tank to the filters and on into the injection pump. It simply transfers fuel from the tank to the injection pump and is a low pressure pump. Low pressure meaning 5 to 15 pounds per square inch.



IMPORTANT TO NOTE: All pumps any type cause a volume of fluid, or air to flow and that pressure is a result of that flow restricted. You cannot have pressure in any system without flow first. Flow under no restriction does not break tubes, it is when the flow is restricted and in this case would be diesel fuel flow that builds pressure inside the system. Pressure allows you to overcome resistive forces and volumetric flow allows continued operation to overcome restriction and rated in time. I have 2 cups per minute of flow through the injection pump for example.



The diesel fuel transfer pump supplies the flow of diesel to the injection pump which produces flow to the injectors through the conduits of the steel injection tubes. These crack, break, rot through, and leak from the copper sealing rings where they bolt onto the injection pump outlet ports and at the injectors themselves. A leaking injection line will cause a diesel engine to run unevenly and possibly dropping a cylinder (a cold cylinder). A cold cylinder if let run that way will wear out the rings on the piston of that cylinder and contaminate the engine oil. Make sure to always run your diesel engine at it's recommended temperature. Every degree less than specified allows more fuel residual inside the cylinder liner and cleans the lubrication off the cylinder walls causing high ring wear.



The fuel injection pump is a high pressure pump which operates at around 1800 to 2500 pounds psi. The fuel injector is a valve normally closed to fluid flow. As pressure builds to cracking pressure (valve begins to move due to pressure building up) at the injector the valve opens and fluid is injected under high pressure into the cylinder.   



Diesel Engine Fuel Systems - Bleeding Air



Below is a drawing of the Gravity supply system for diesel engines. The gravity flow systems are found on some tractors, generators and stationary systems. This gravity method actually reduces the need for a separate pump that transfers the diesel fuel from the supply tank to the fuel injection pump. A fuel injection pump has a filter screen at the inlet of fuel and a charge pump to the pistons inside the fuel injection pump.



Air needs to be removed from the fuel flow within the tubing and cavities that are connected to the fuel injection pump. As well the outer case housing of the fuel injection pump holds a supply of fuel for the inside of the pump parts. All sealed systems flowing diesel fuel to a fuel injection pump need to be free of air or the engine will stop and not start until the air is gone. 



The only disadvantages of the gravity system without a transfer fuel pump is that the level of diesel fuel inside the tank determines the pressure available to flow through the suction filter. If the tank is low and the filter is dirty the fuel low pressure could cause the engine to surge while running and finally quit, and the need to bleed air is then required to get the engine going again after re-filling the tank with diesel. Another disadvantage is needing to lift the 5 gallons of diesel up to a higher location. Even a small tractor that has the gravity fuel supply system takes more effort to fill than the other two diesel fuel systems when an electric fuel transfer pump is not available.

The gravity fuel system starts at the tank and inside the tank is a little pipe and usually a screen at (letter a.). This point is the most common cause of diesel engines surging and then finally quitting and then hard to bleed air from the system. Inside the tank develops a sludge and it appears as a black slick moss like sludge. This will restrict the diesel fuel going to the fuel transfer pump and then to the inlet of the fuel injection pump. When this screen and pipe is plugged the engine will run fine and just start slowing down and quit, like a wind up toy running out of spring tension.

When a diesel engine begins to surge and then runs normal at steady RPM and then engine goes to lower idle then higher (surging) it is likely a fuel supply restriction at a. in the drawing. 

The suction diesel fuel supply system is essentially the same in operation as the gravity system. The difference is that the supply tank is lower than the rest of the components supplied with fuel in the system. In this bleed procedure the suction filter bleed nut 1. is not opened for the procedure. When a system that uses the suction supply method surges and needs bleeding, it usually has the point a. partially plugged. That situation produces more suction in Line 1. & 2 which in turn may allow air to enter where hose clamps are used.

Working on the pressure method is easiest for the mechanic, turn on the electricity to the pump wires in the fuel tank and open each bleed valve letting air out until a clean flow of fuel is exiting the bleed nut holes and tighten each one in a sequence as they are bled.

Glow Plug & electricity added 2 seconds & Relay solenoid control.



Glow plugs ! I get the same thing over and over here, a diesel engine will not start and the person has said that they were told they did not need glow plugs in Hawaii. Glow plugs have one job and that's to heat air or when hot vaporize diesel fuel to a smoke.



Most diesel engines that are in a warm environment start fine without glow plugs working, but the situation is that the diesel fuel will not blow up unless it reaches a specific temperature and spontaneously combusts.



Here are some shop facts that need to be understood when trying to figure out why the diesel engine will not start and run smoothly.


1) Fuel is the #1 cause of most all diesel fuel engines failing and not  starting.


2) Overheating will shut most engines down, check engine coolant in radiator. Lack of oil pressure will shut most engines down, check engine oil. Lack of battery voltage at the fuel shutoff solenoid will stop fuel from being injected into the cylinder head area, check for voltage at the fuel pump shutoff solenoid.


3) Start checking fuel problems by going first to the fuel injector tubes and loosen the injector tube bleed nuts at the cylinder head. Use the starter to rotate the engine and hold the throttle lever all the way wide open. As the engine is rotating fuel needs to come out of the tubes without air bubbles.


4) If fuel is not coming out without bubbles as the engine is rotated at the injector tube bleed nuts then step back to the fuel injection pump fuel inlet tube and take it loose at the pump. Pump the transfer fuel pump and see if the fuel will come out of the fuel injection pump inlet tube.


5) If fuel is present and flowing out of the injector tube bleed nuts then tighten them back up one at a time while attempting to start the engine. If white smoke is coming from the exhaust there is plenty of diesel as the engine is cranking.


6) Check the glow plug circuit for electrical input in volts DC, test each glow plug with the OHM setting on your meter.


7) Use a wire to jump from the +12 VDC post on the battery and one glow plug, usually all glow plugs are commonly grounded to the case of the glow plug and screw into the cylinder head which is grounded to the battery - terminal post. What you will be looking for is a spark when you touch the jumper wire end to the glow plug wire and hold it there for at least 5 seconds or until your wire get's to hot and then try to start the engine. If it starts the glow plug circuit needs worked on and repaired.


8) If there is white smoke as you are trying to start the engine and the engine will not start then pour a little gas on a shop rag and hold it near the air intake as rotating the engine to start. This should make the engine fire off a couple cylinders and then start.



Diesel fuel will blow up at a specific temperature and I think an old rule of thumb is around 485 degrees F. That means if you had a hot plate and put a few drops of diesel fuel on it as soon as the temperature got around 500 degrees the fuel would just start fire by dropping it on the hot plate with no matches or flame involved to start it burning.



This is called spontaneous combustion of the fuel. So a diesel engine has no spark plugs because diesel fuel is difficult to light compared to gasoline. A diesel engine uses the compression of the air and fuel to make it self ignite and burn. Every pound of compression in psi. will raise the air temperature by 2 degrees F. and so the diesel piston compresses the fuel air mixture until it self ignites.



So the compression pressure inside the cylinder heats up the mixture of diesel and air to a burning temperature. When the air temperature is lower than usual 60F and below and the humidity is high the air cools the fuel and floods the engine if it rotates without starting immediately. Every time the piston comes to the top of the stroke the injector squirts a shot of diesel inside the cylinder, if the engine takes several times to start, more fuel is squirting in further cooling the fuel air mixture inside the cylinder and this floods the diesel engine out with excessive fuel.



The glow plug heats up the air to a higher temperature to assist in firing the diesel inside the engine cylinder. Think of it in this simple way, you have a water heater that turns on when you need hot water to take a shower. Your burner is set to heat the water just perfectly in a specific amount of time. You set the gas flow to heat the water that is normally around 60 degrees to shower in 60 seconds and keep it flowing at that rate warm enough to shower.


Now it is hot outside and the sun has warmed up the water in the inlet line to the water heater to 80 degrees F. When you start the shower water it get's hotter quicker due to the added inlet temperature of the water source. Now it gets real cold and drops the inlet temperature to 40 degrees and the water for the shower never get's hot enough to shower.



That's how the Glow plugs help the engine to start quicker, the added heat to inlet air temperature. The reason this is so important is that high engine ring wear will result in a lower compression pressure as a result of the extra diesel fuel wiping down the cylinder from oil acting as a cleaning solvent instead of a fuel mixture.

Since the diesel engine relies on compression pressure to develop heat to fire off the diesel fuel air mixture inside the cylinder every pound of compression less than new would have will result in less fuel being burned inside the compression chamber. This in turn will wear the internal components faster as well as translate the inefficiency into friction. Make sure your glow plugs are always used if you have one of those engines that have them.