Thursday, September 2, 2010
Detroit Diesel 2-Stroke Series 71, 53, 92 Engines
I put this video on Youtube. It is an Oliver 1950 farm tractor, powered by a 4-53 Detroit Diesel. This is one of the old 2-stroke Detroit Diesels that used to be so prevalent in generators, large boats, garbage trucks, and lots of road tractors. Although the Detroits are very popular, they are unusual in that nearly all competing diesel engines in their size range are 4-stroke designs. Bigger diesel engines, such as those powering ships, are often 2-stroke.
A 4-stroke diesel engine has one or two intake valves and one or two exhaust valves per cylinder, located at the top of the combustion chamber. These valves are actuated by a camshaft, which is geared to turn at half of the crankshaft RPM. It works like this, one stroke at a time:
1. The cam opens the intake valve as the piston moves down, drawing air in through the intake valve.
2. The intake valve closes as the piston moves up, compressing the air. Because of the high compression ratio, the air gets very hot as it is compressed. A few degrees of rotation before the piston reaches top dead center, the injector introduces a shot of fuel to the combustion chamber. The hot air ignites this fuel, which begins expanding.
3. Because the valves are closed, the expanding column of gas has nowhere to go but down, pushing the piston ahead of it. This is the power stroke.
4. When the piston reaches the bottom of its travel,the exhaust valve opens. The piston travels back up, pushing the expended gas out through the exhaust valve. When it reaches the top, the exhaust valve closes, the intake valve opens, and the cycle starts over.
The Detroit Diesel 71/53/92 series 2-stroke engines have a cam too, but it is geared to run at the same RPM as the crankshaft. The cam actuates two or four exhaust valves per cylinder, which are located at the top of the combustion chamber, same as in a 4-stroke engine. But there are no intake valves. Instead, there are intake ports cut into the cylinder wall at the bottom, where they are covered by the piston except when the piston is at the bottom of its travel. Here is a description of operation; in this case we will begin with the power stroke:
1. The exhaust valves are closed and the piston is being driven downward by the expanding gas column. As it nears the bottom the exhaust valves are opened by the cam, and at the same time the intake ports are uncovered by the piston. There is a blower on the side of the engine, driven by a gear from the crankshaft, and this blower blows air into the cylinder via the intake ports. Because the exhaust valves are also open, the incoming air pushes the spent gas out the exhaust valves.
2. The piston starts its upward travel, covering and shutting off the intake ports as the cam allows the exhaust valves to close. Now the combustion chamber is closed, so the piston compresses the air within it, which becomes very hot. A few degrees of rotation before the piston reaches top dead center, the injector introduces a shot of fuel to the combustion chamber. The hot air ignites this fuel which begins expanding, bringing us back to the starting point.
It is worth pointing out that 4-stroke diesels have a high-pressure fuel injection pump which greatly compresses the fuel and distributes it to the cylinder that is ready for it. This pump has precise internal clearances, and is subject to wear if the fuel lacks the necessary lubricity, as in the current ULS, sulphur below 500 parts per million US diesel fuel requirement. It also is subject to damage from water, dirt or any other contaminant. Alternative fuels like waste vegetable or motor oil cause problems too, to varying degrees depending on the design of the pump; but well-processed biodiesel is injection pump friendly; more so in fact than ULS diesel. These pumps are key to the operation of 4-stroke diesel engines, and cost thousands of dollars to repair if worn or damaged.
Current highway diesel engines lack this pump, having instead electrical plunger-operated injectors which are fed fuel from a moderately pressurized, common rail. These injectors are fired by a signal from a computer. If an injector develops a problem, it may be simply replaced.
Detroit Diesel 2-stroke engines also lack the high pressure injection pump. They have a moderately pressurized common fuel rail feeding the injectors, which are fired (and high pressure developed within) by camshaft. These injectors may be simply replaced if they develop a problem, and they are also individually rebuildable. This factor, combined with the very heavy-duty construction of the engine, makes the 2-stroke Detroit Diesel an extremely reliable, long-lived engine.