Boating complicates our lives, and much of our mutual frustration comes from inside the engine room. Not knowing how a system was designed to work, and why you should not do certain things, can cause you hours of torment and to loose excess amounts of cash for no good reason. Since over 50% of new Diesel boat owner have never owner a Diesel boat before, there is a steep learning curve to be mastered. The sooner you master it, the smarter, safer and better boater you will become.
One of the least mentioned, and most easily fixed problems with diesels engines is knowing why not to run them a low idle speeds. Your understanding of how and why the diesel works will lead you to understanding how to prevent the serious damage that can be caused by idling an unloaded diesel engine. Diesel manufacturers are just a little less than clear on the pitfalls of idling a diesel engine unloaded, here is an example.
In their manual, one diesel manufacturer simply states: “Unnecessary idling of an unloaded engine should always be avoided.” But gives you no further information, similar wording is given by almost all diesel manufactures somewhere in their manuals.
Lets first talk about how a diesel actually “fires”, there are no spark plugs in a diesel engine, the spark comes from injecting pressurized diesel fuel into the cylinder and having that fuel compress until the pressure of the upstroke of the piston causes it to explode. The explosion is typically mechanically timed (on non electronic engines) and is predicted to occur under certain ambient and load conditions. These conditions are altered somewhat by the engine, with the use of glow plugs that we use in cold situitations to pre heat the inside of the cylinders to aid in the initial combustion. We also, depending upon temperature, pre heat the diesel engines intake air to aid in the initial combustion. As the diesel runs, the internal explosions and friction from the rings and other mechanical surfaces heat the engine to its predetermined, constant operating temperature. This temperature of the engine block is controlled by the engines thermostat, which regulates the cooling waters circulating through the engine block.
So far so good, the engine has started and is now idling at the dock, as it idles, the temperature of the engine block is rising to its pre determined operating temperature. BUT, in many diesels to achieve this temperature we must be at a speed above idle! AND to achieve the complete fuel burn we must have a load put on the engine, (propeller turning, pushing the boat). Here’s the problem, we need our engines to idle at low RPM in order to shift them in and out of gear, we need them to idle to maneuver into and out of our docks, we need them to idle for trolling while fishing. But many of the engines out there today idle in the 550 to 600 RPM range. Great for docking, but too low of an RPM to achieve the internal temperature needed for complete combustion. “So what,” you may say.
Incomplete diesel combustion causes that “white smoke” upon startup. The white smoke is actually a combination of unburned, and partly burned fuel droplets, this condition is typical, until the walls of the combustion chamber gradually heat up, and should last no more than a minute. Continued operation in this idle range will continue the incomplete combustion of diesel fuel and will cause:
1.- Carbon build up.
2.- Low compression due to sticky piston rings.
3.- Piston rings not sealing properly.
4.- Choked valves.
5.- Increased heat on the piston surface leading to a “holed piston”.
6.- Unburned diesel fuel dripping past the pistons wiper ring, into the lubrication oil, thus contaminating it and increasing the wear to all of the interior bearing surfaces of the engine.
7.- Build up on the injector spray nozzle, causing an incomplete spray pattern.
Even after starting, the temperature in the combustion chamber may still be too low to bring the block up to temperature, you can monitor this yourself on your own engines temperature gauge and judge for yourself your individual conditions.
What can I do?
Have your crew standby prior to starting the engine, allow at least 30 to 60 seconds for the engine to “prove itself” before casting off. Use a Cetane booster in your fuel, the Cetane rating of diesel fuel is similar to the octane rating of gasoline, it’s a measure of how readily the fuel starts to burn, the higher the Cetane number, the faster the diesel fuel will start to burn. Cummins for instance tests and certifies their engines using 44 Cetane fuels, the highest Cetane rating I have ever seen at a marine diesel pump was 40 Cetane. To make up the 4 points of Cetane I have used Racor’s diesel conditioner plus for the past four years, it adds 4 Cetane points to the fuel, improves lubrication, combustion and stabilizes the fuel. There are other diesel conditioners out there, I just happen to try and like the results from this one, and one gallon treats 2560 gallons of diesel, I’ve used four gallons so far and have no regrets. After you are clear of your marina, and can safely raise the RPM do so, 900 RPM in most cases will bring the engine up to temperature, remember to watch YOUR temperature gauge so you will know what YOUR particular engines needs. Manatee zones, slow speed zones, waiting for bridges, make some low idling unavoidable. But, if you’re running with twins, you can always shut one down, and bump the RPM up on the other. Fishing at low speeds requires the installation of a trolling valve on the engines transmission. Think of this as a bypass valve which allows the engine to turn over at a fast speed but allows the transmission to “slip” and slow the propellers speed. Diesels are made to run them hard, in their cruising range, for pleasure craft that is typically 10% under their WOT, (Wide Open Throttle) RPM. Diesels in the pleasure craft category are rated to only run at WOT one out of every eight, or one out of every ten hours of operation at WOT, depending upon manufacturer, but that another article. If you have an engine that has sooted up from excessive idling, just take it out for a run and within 2-4 hours of cruising you will generally be all cured.
Our newest (common rail) diesel engines typically use a high pressure fuel pump, putting out in the neighborhood of 27,000 PSI of pressure and in conjunction with computer to controlled injection timing, that at low RPM, pulses a small amount of fuel into the cylinder to act as pre heat event, and then is followed by a pulse of a larger amount of fuel into the cylinder for the power stroke. The pre heat event is just enough to raise the inside temperature so that the power stroke event can fully burn the injected fuel. The computer controls the timing of the fuels delivery to the injector, as well as the amount of time the injector is allowed to remain open at various RPM’s. This technique provides for very complete fuel burns and very quiet operation at low RPM and therefore the newest common rail engines are not prone to the problems encountered in our previous more mechanically controlled diesels.
Additional contact Information:
Your local engine service shop
Captain Jim Clausen, MV Summer School
CaptainJim@maxumowners.org
