|Marine Forensic Technicians|
James R. (Randy) Renn, SAMS® AMS®
Hello well rounded boaters! Happy winter weather boating. This article will discuss some of the least attended, most complex, and perhaps least understood yet most important components we have in our diesel engines. These items are in constant use while under power, cycle millions of times a season, work within thousands of a second accuracy and are in real danger of decay while laid up. They are mechanical diesel fuel injectors. We perhaps know where they are located however they get almost no consideration in way of maintenance. Point of fact there is very little hands on maintenance that can be performed on the injectors themselves outside of a repair shop. We will discuss some principals, parts and pieces and those areas where we may affect maintenance well before a failure. It will be paramount to remember at all times that fuel injectors are truly fine instruments as well constructed as any self winding wrist watch and much more delicate in the wrong environment. Because we do not make direct efforts towards our injectors can certainly be appreciated as they stand a mystery and are very rarely brought into the open air. After all one never hears "how is your injection gear these days?" With that let us jump into some of the marvels and the myths of diesel fuel injection.
Your diesel engines injectors find their history in principal from around 1895 when injection was made of coal dust or tar oil and eventually crudely refined petroleum. As fuels have improved so has the machining of injection equipment. It is not incidental that older primitive injectors suffered from the same demons that can plague modern systems. Those being fuel quality, water and small contaminates. Same story, different century. As a side note, Rudolf Diesel, the mistakenly reported father of the system predicted near his passing in 1913 that "vegetable oils" would become important as a fuel source at some point-sounds like Bio-diesel that is in use today.
Diesel injectors have either fuel supplied by very high pressure pumps or are high pressure pumps themselves. Pressures in most diesel engine injectors that are not "common rail" engines will be in the 3,200 to 5,000 psi, yes that is correct 5000 pounds per square inch! Common rail engines which we will not discuss here can have staggering pressures up to 29,000 psi. The purpose for such high pressure is to atomize the fuel to a very fine mist, very quickly, within micro windows of timing but in varying amounts dependent on speed and load. Additionally the fuel pressure must remain almost exactly the same at all speeds. The injection pump forces fuel at great pressure towards the injectors, the pressure overcomes the tremendous spring resistance within the injector-see photo. The injector "pops" open for a micro second releasing pressure. The fuel is injected into the engine combustion area where the engines pistons have compressed-heated air to such a point that when the mist of fuel hits it the combination explodes. The injector then slams closed. This rapid opening and closing are crucial in proper injector performance. Wow, a lot to do from something that is purely mechanical.
With that level of pressure come several necessities. First, heat must be dissipated. Diesel combustion temperature can easily be 1300-1600 degrees and injector tolerances are very tight. One end of the injector lives in the combustion part of the engine and one parted is wrapped in the engines cooling system, 170 degrees, while the rest of the injector is exposed to open air. Different levels of expansion going on along the injector length. Many injectors have a fuel return line that delivers unspent fuel back to the fuel tank and carries away considerable heat with it. The heat situation has a second effect in way of the fuels capacity to keep the moving parts of the injection system lubricated-called lubricity. This means the fuel must be maintained in a fresh state at all times to keep those additives working that are important to lubricity-heat makes matters worse. Next, diesel fuel is measured in something called Cetane rating-very complex and calories-just like food. In order for the diesel to run properly and fuel to burn correctly in the heat of the combustion chamber a precise amount of very good fuel must be added in a precise time frame. If the fuel will not burn correctly in the heated air of the combustion chamber you add throttle giving more fuel the engine heat cannot burn all the fuel and some is left as ash or varnish that helps to contaminate your fuel injector(s).
As the photos indicate an injector can come in many if similar shapes and are basically made up of an injector body-upper part and a injector nozzle-the really shiny parts-lower part. The nozzle and needle are very, very finely machined. In fact with a new nozzle the fit is so fine if you were to hold a new needle in your hand and let in warm in may not fit into a cool nozzle due to expansion. The injector is connected to the rest of the fuel system by a fuel delivery line and likely a fuel return line-much smaller line which may have "hollow bolts" through it-see photos. All parts of the system must be kept extraordinarily clean at all times and at no times should any injector internal part be handled with dry hands or be dry themselves. Parts must be kept coated with "fuel" so the machined parts do not become etched with salts from dry handling.
So now the questions, when, why and how do I service my injectors?. Your injectors are subject to damage from contaminates and water. If you note your engine is smoking, weak or very noisy you may have an injector issue. A quick way to determine if an injector is not working is to "short the injector" by "cracking it open while the engine is running. "Cracking" means to open the injector main feed line at the injector using a proper "line wrench" when the engine is at idle-see photos of the correct wrench as opposed to a normal spanner. Opening the injector as little as a quarter of a turn while wearing goggles, gloves and keeping the area covered with a cloth has the effect of turning it off. If fuel leaks and the engine shows no slowing or change then the injector is not working correctly. If the engine was making loud hammering noises and when the injector is "cracked" the noises stop then the injector is "open". In either case the injector will have to be removed. If the "cracking" does have an effect move on to the next injector. This operation is a bit messy but you will lose very little fuel. I suggest having your valves adjusted as part of this test. In general if one injector is weak all should be serviced. Suggest service every thousand hours, when fuel has aged badly or been heavily contaminated by water.
If you have spare injector about the ships stores you may replace a weak unit by removing the fasteners-see photo or unscrewing it from the cylinder head. Most injectors break free easily with gentle prying and penetrant. We like a Penetrant called "B-Laster" although fresh diesel fuel works well. Injector lines and line keepers(little braces) must be loosened before any is removed and lines MUST NOT BE BENT!! Once any line is removed all openings must be kept covered-see small red plastic caps on injector photos. Note: The reason the lines are twisted and serpentine is to keep them the same length-cocktail conversation. When replacing injectors care must be taken that the copper heat/seal washer is replaced. This means that the old one must be removed from the cylinder head if it did not come out with the injector. You should consult your engine parts manual for the placement of seal rings. The fasteners will have a "torque setting" and a torque wrench must be used to ensure the fasteners or injectors do not apply too much pressure to the cylinder head. Servicing of injectors is nearly impossible outside a shop because once apart in order to correctly reassemble them a "pop tester" tool is needed-see photos, also note how high and dangerous the pressures are. Do not remove an injector and put it back on a line in open air to turn the engine over and test it. This results in a very fine high pressure mist that if touched while working will puncture skin and cause serious blood or tissue poisoning. We have had some success using an injector cleaning kit-photos when in desperation and those moments do come about on occasion.
Once the injector operation is complete the units must be "bled" and the engine test run. You should have changed all the filters, separators and crush washers during this activity. Fresh fuel should be a priority as should have all units serviced at the same time as "pop testing pressures" from one shop or builder will vary. All units should come from one house-supplier.
Diesel fuel injectors are quite expensive and one should not be hesitant to use rebuilt items where possible. Remember cleanliness is paramount! The first step in injector service is proper fuel, fuel that is not badly aged, clean air and unrestricted exhaust out and keep those valves adjusted. If you have not engaged a technician to inspect your engines timing is then doing so will eliminate some worry and very likely smooth out a smaller engine, improve larger engine performance and longevity to any. Adding additional fuel filters with bypass system and water alarms would be another positive investment.
James R. Renn, SAMS, AMS-YSCE, MIIMS
|Go to main page.
Copyright © 2008/2019 Marine Forensic Technicians, Stevensville, Maryland - email@example.com
Installed March 15, 2008, Last Update January 6, 2019 - Hosted and Maintained by Don Robertson