~ Diesel Fuel Injection

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ROUGH DRAFT

DEMYSTIFYING THE FUEL INJECTION SYSTEM
Engine stalling, lack of use, corrosion, water, polluted fuel and other contaminates all contribute to injector and injection pump failure.
STORY AND PHOTOS BY LARRY BLAIS
In the early days of the internal combustion engine, igniting the fuel once it was in the combustion chamber was a major hurdle faced by designers. When Rudolph Diesel noticed that, in refrigeration systems, the temperature of the medium would rise when compressed, he theorized and patented the idea that the heat generated by compression could ignite the fuel. Five years later, he proved his theory but his design revealed another major problem inherent in all high compression engines. Once ignited, the fuel might burn so rapidly that the gasses inside the combustion chamber would be forced to expand faster than the speed of sound, generating a shock wave that would leave a path of destruction in it’s wake.
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Note how the detonation shock wave has literally pulverized the top of this piston.
Engines had to be very heavily built to survive this shock wave and thus, large and slow turning. It wasn’t until the development of the high pressure fuel injector by Bosch that the full potential of the compression ignition engine could be realized. By injecting the fuel in precisely shaped and sized droplets that would ignite and burn off the surface of each droplet, the rate of burn could be controlled and the creation of shock waves avoided.
Engines could now be built much lighter and produce more power. As the years passed, the fuel and the engines have been even more refined. A modern diesel engine is a truly marvelous piece of machinery. Let’s do the math. Inside an engine turning 3,000 rpm, each piston starts moving from a dead stop at one end of the cylinder bore, accelerates to the midpoint, decelerates to a complete stop at the other end of the bore and then does the same thing going the other way; 6,000 strokes per minute. That’s 100 strokes per second! In a four stroke engine, the fuel injector would have metered and injected 25, 50, 75, 100 times in those same four seconds. That’s 90,000 injections in an hour. If the engine has four cylinders and burns only 3.78L (1 gal) of fuel an hour, each injection would deliver the minuscule amount of .945/90,000 of a litre (1/90,000 of a quart).
Rx for Injectors
All modern fuel injectors are pretty much the same internally. At the tip is a valve seat and valve that is opened by fuel pressure generated by the injection pump against the little piston and held closed by the spring. Some fuel leaks past the little piston, lubricating it, and is then returned to the fuel tank. The amount of pressure required to open the valve is often called pop pressure. The direction and intensity of the spray is called “pattern.” The pattern must match the shape of the combustion chamber to burn properly. This is sometimes called “indirect injection” and this injector is pretty much self cleaning. Pre-combustion chambers were developed in an effort to help engines suppress and/or tolerate stronger detonation shock waves by moving the initial combustion area away from vulnerable parts such as pistons, rings, valves and head gaskets. Forming a small, reinforced, swirl chamber deep in the heavy casting of the head did this quite effectively. The down side was harder starting. The air being heated by compression would have to travel past so much cold metal that enough of its heat was absorbed by the cold metal that it could no longer ignite the fuel. This problem was solved by the installation of little electric heaters, commonly called “glow plugs,” in the pre-combustion chambers. How long these little heaters are activated before starting is attempted depends on the actual temperature of the engine more then anything else. In colder climates, other starting aids may need to be incorporated such as block heaters or air intake heaters. Since engines with pre-combustion chambers are built with lighter pistons, valves and head gaskets, NEVER use starting fluid in these engines. The starting fluid may ignite on top of the piston before the piston is fully up, causing extensive damage.
All diesel engines will accumulate some soot in the combustion chamber while being warmed-up. The gasses inside the combustion chamber need to reach almost 538°C (1,000°F) to sustain complete combustion of the fuel. A diesel engine idling at the dock will rarely ever attain these internal temperatures, even when the water temperature gauge reads 76.6°C (170°F). Nor will it get warm enough internally while idling at the dock in gear, so the longer it idles, the more soot it accumulates. To help keep injectors clean, idle at the dock just long enough to dependably, without stalling, put the engine under the load required to maneuver out of the berth. [Ed: I prefer an engine warm up of at least 10 minutes, rather than chance a stall.] Once past the breakwater, slowly increase the throttle as the engine continues to warm up until cruising speed is attained. Sailors should refrain from hoisting their sails until after their engines have had a chance to burn out the soot under load for 15 to 20 minutes. Doing this will go a long way in preventing many expensive soot related problems later. A word of caution: diesel engines should never be operated at high throttle if they are over-propped or the boat bottom is dirty enough to lug the engine.
Fuel Supply
Two types of injection pumps, the other major component in the fuel injection system, are most common on modern marine diesels. Pipes to the injectors being all in a row distinguish a multiport jerk pump. The crankshaft through a set of reduction gears drives a small camshaft. The lobes on the camshaft push the little pistons up against the springs forcing fuel through the pipes connected at the top to the injectors. The governor and the throttle linkage determine how long fuel is sprayed into the engine during each injection. By pulling the throttle back, the duration of each injection can be shortened to the point that the engine can idle under no-load. Each little jerk pump
“Sailors should refrain from hoisting their sails until after their engines have had a chance to burn out the soot under load for 15 to 20 minutes.”
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Injector removed from a 1986 Volvo diesel shows signs of corrosion and soot build-up. Calibrated to very close tolerances, injectors are easily plugged by polluted fuel, water, corrosion and other contaminates. Always wear gloves or hold the injector with a rag when servicing as moisture from fingers or hands can destroy them.
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This is what a rotary injection pump looks like inside. Instead of a separate pump piston for each cylinder, it has one main high-pressure pump and a rotating selector valve that delivers the fuel to each injector.
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piston can be adjusted so that each cylinder does its equal share of the work. If these are out of balance, one or more cylinders may be over-fueling and trying to do most of the work, causing them to lug and even damage themselves.
A rotary pump can most easily be identified because all the injector pipes come out of the pump in a circular pattern, instead of all in a row. One of the few benefits of sulfur in diesel fuel was that it acted to maintain the uniformity of lubricity in the fuel. Because the (top) Pipes in a row leading to the injectors indicates a multiport jerk pump. (bottom) This inside view shows the cam across the bottom driven by the crankshaft through a set of reduction gears. Cam lobes push the pistons up against springs forcing fuel through the pipes to the injectors.
rotary-type pump utilizes numerous seals and sealing rings, it has suffered the most from the advent of low sulfur diesel. New materials have been developed for these pumps to help them tolerate low sulfur fuels.
Regardless of what system is utilized, one thing holds true, these are very precision, close tolerance parts that can be damaged very easily by abrasion from contaminants in the fuel or from galvanic corrosion or etching from acidic by-products of a microorganism in the fuel. They can rarely be serviced in the field and should be serviced by a qualified technician. Keep them healthy with clean, fresh fuel and run the engine long enough, hard enough and frequently to burn out the soot.
About the author: A master mechanic, master shipwright and marine surveyor, Larry Blais has operated boatyards for more than 30 years. He teaches classes for the United States Coast Guard, Havorn Marine Survey and Shipwrights’s School, University of Washington’s Sea Grant program, and hosts workshops in diesel care for the Northwest School of Wooden Boatbuilding.
P
Note how the detonation shock wave has literally pulverized the top of this piston.
P
The above injector is designed for an engine where the fuel is injected into a pre-combustion chamber.
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The tip of this injector is different, as it’s designed to inject fuel in a broader pattern into the main combustion chamber of a direct injection engine. The little discharge orifices on this injector can easily become clogged by soot.
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Don’t For Cold Starts
Never, ever use starting fluid in marine diesel engines. Extensive (and costly) engine damage may occur should the starting fluid ignite on top of the piston before the piston is fully up. — LB
P
The tip of this injector is different, as it’s designed to inject fuel in a broader pattern into the main combustion chamber of a direct injection engine. The little discharge orifices on this injector can easily become clogged by soot.
P
Injector removed from a 1986 Volvo diesel shows signs of corrosion and soot build-up. Calibrated to very close tolerances, injectors are easily plugged by polluted fuel, water, corrosion and other contaminates. Always wear gloves or hold the injector with a rag when servicing as moisture from fingers or hands can destroy them.
P
(top) Pipes in a row leading to the injectors indicates a multiport jerk pump. (bottom) This inside view shows the cam across the bottom driven by the crankshaft through a set of reduction gears. Cam lobes push the pistons up against springs forcing fuel through the pipes to the injectors.
P
This is what a rotary injection pump looks like inside. Instead of a separate pump piston for each cylinder, it has one main high-pressure pump and a rotating selector valve that delivers the fuel to each injector.
P
On some engines, especially older, larger engines, the injector pump is actually part of the injector and is mechanically actuated through a linkage by a lobe on the engine cam. A throttle rack connects the throttle linkage and governor to each of the injectors to control injection duration and thus engine speed.
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