Run Your Diesel at the Right RPM

Many powerboat cruisers operate at speeds and loads far below the levels that engine manufacturers expect. To maximize fuel economy, many of us cruise at displacement speed (less than 1.34 times the square root of a boat’s waterline length). That kind of speed requires relatively low rpm, and the load on the engine might be below 25 percent of its capacity.

On older engines, this kind of sedentary life can be harmful. But what about with modern common rail computer-controlled engines? Does speed still matter, or are such problems no longer a concern?

In the spirit of those who have busted myths throughout time, we set out in search for a definitive answer.

The Perils of an Easy Life

Many passagemakers are fitted with engines that have just enough extra horsepower to stem a tide or buck the wind, and that are never intended to go much faster than their displacement speed. They are fuel efficient, and they allow for great range. Most of the time, these engines are operating at 70 percent to 80 percent load at cruising speed.

Other boats pack on larger, higher-horsepower engines that let them push out of the displacement hole. This extra speed can be a huge benefit when you need to make it to a harbor before a storm, or for going farther when you need to get anywhere faster.

When these higher-horsepower boats run at slow speeds, the light load on the engine may cause serious side effects. All boat owners pay attention to high engine temperatures, and we go to great lengths to keep our engines from overheating. But what about the opposite scenario: Can an engine be too cool?

While gas engines use a spark to ignite the fuel, diesel engines rely on high compression. Injectors on older diesels “pop” at a specified pressure, allowing a measured quantity of fuel into the cylinder. At high speeds and high loads, the engine generates enough heat to properly burn all of the injected fuel.

At low speeds and light loading, cooler operating temperatures reduce heat in the cylinders. The cooler cylinder temperatures result in unburned fuel, which mixes with air to form soot—which looks a lot like burnt tar. This unburned fuel gets pushed out into the exhaust, where it can coat the backs of the valves and the exhaust system.

The buildup is called coke (the process of it forming is called coking), and it can start a downward cycle for the engine. Excessive coke can build up inside the exhaust system to the point that it restricts the exhaust flow, creating excessive back pressure. It can coat turbo components, loading the blades and causing failure. This buildup also fouls intake and exhaust valves, preventing them from sealing properly and weakening engine performance.

This carbon buildup can also clog the tips of the fuel injectors, plugging fuel delivery or creating less-than-optimal spray patterns. Note that the same coking situation, sometimes called wet stacking, arises within a mechanically injected diesel generator that is run without enough load. This concern can be especially noteworthy if the generator was improperly oversized when it was installed. Some boats mitigate the problem by installing a smaller generator and a larger one. When light loads are anticipated, the smaller generator is run, and when it’s 90 degrees out, all A/C units are running, and there are five loads of laundry to wash and dry, the larger generator is brought online.

Other problems can occur when the engine doesn’t run with enough load. Rings around each piston fit tightly against the cylinder walls, and they contain the pressure from each compression stroke. These rings rely on heat to expand properly. Lightly loaded engines may not heat enough to seal the rings, allowing unburned fuel past the rings and into the oil pan, where the diluted acidic mixture can lead to internal corrosion and loss of oil lubricity.

Normal combustion creates some amount of water vapor, which can end up in the engine oil (in addition to the moisture that forms in the engine from normal condensation when the engine is not in use). According to Dennis Bachelder, senior program engineer at the American Petroleum Institute, bringing the oil temperature up above 212 degrees Fahrenheit will cause this moisture to evaporate. On a lightly loaded engine, the engine oil may not come up to this temperature, leaving moisture in the oil that could harm internal components and cause problems.

Obviously, there is a balance between an engine running hot enough to fully combust the fuel and running so hot that it overheats. Carefully engineered passages carry coolant close to the cylinders, where it can absorb the heat and move it away to be exchanged with cooler seawater. Thermostats shorten the coolant path so that it heats up quicker, and then open the full circuit to keep the coolant temperature near 180 degrees Fahrenheit (and not much over 200 degrees). If things are too hot, then the coolant will boil, and the engine will overheat. Again, problems.

Engine oil will run hotter than the coolant, somewhere between 230 and 260 degrees Fahrenheit if the engine is fully loaded. If it is not fully loaded, and especially if the weather is cool or if the water temperature is cold, the engine oil will run much cooler.

Stoichiometric What?

For peak efficiency, an engine must use as much of the fuel’s energy as possible. This efficiency can be measured as the ideal fuel-to-air ratio that results in complete combustion: the stoichiometric ratio, as experts call it.

As technology improves, and as fuel economy and excessive emissions become more important, engine manufacturers devise ways to decrease the wasted fuel. An important improvement has been computer-controlled injectors with a pre-pressurized supply. In these engines, the fuel injector pump pressurizes the diesel supply at 20,000 to 40,000 psi and then feeds it into a manifold that’s typically called a rail. It is called a common rail because it feeds all the fuel injectors.

The computer can trigger the injectors to open for various amounts of time. An older injector will open one time for each cylinder stroke and will always release the same amount of fuel, while a modern, computer-controlled injector might open seven times per cylinder stroke and in varying amounts with each pulse. This latter injection pattern allows engines to run much smoother and with less vibration. Sensors throughout the engine feed information into the computer, which is programmed to adjust the fuel injection based on the environmental inputs. With a mechanically injected fuel system, the engine may only be capable of running at peak efficiency at around 80 percent load because the cams that control the fuel pump can only be machined with one profile.

Not only are computer-controlled common rail engines cleaner, smoother-running and more fuel efficient, but the more precise fuel injection has virtually eliminated the problems associated with running at low loading. Coking is greatly reduced because the computer will reduce the fuel to match the load.

Your Current Situation

To ensure that these problems won’t affect your boat, start by identifying which type of engine you have. People often ask for a specific year, but things are not that simple. You need to know if you have a mechanically operated engine or a common rail with an ECU (computer) that controls performance. 

As regulatory agencies settled on a push for cleaner, more efficient engines, they implemented the requirements in escalating stages of compliance, beginning with Tier 1 and moving to Tier 4. The timing of the tiers depended on the horsepower of the engine. Common rail fuel systems started showing up in boats in the mid-2000s and were popular by 2015, when the Tier 4 emission standards came into effect for new marine diesels.

There are still exemptions for retrofitting a mechanically fuel-injected replacement engine, and a few companies sell factory-refurbished older diesels as well as new, smaller-displacement, nonelectronic engines.

If you are not sure about your engine, find the serial number on the engine identification plate and call a dealer.

Mechanically injected diesels can be extremely long-lived. They’re simpler, so they can be easier to work on, and the lack of electronics can be attractive. Older mechanically injected fuel systems use a high-pressure fuel pump that is timed to the piston, releasing a charge of fuel via a spring-loaded injector. As mentioned earlier, this injector can only fire once per piston power stroke, and the regulation of fuel is much less precise, but it can be reliably simple and will never require a technician with a computer connection to diagnose a problem. Additionally, replacement cost for a mechanically controlled injector might be five times less expensive.

That simplicity does require a different operating recommendation. The engine should be brought up to at least 75 percent load for 30 minutes or so in a day’s run. On an older engine, it might be difficult to determine load—a general rule would be to run at 10 percent to 15 percent below maximum rpm. Running at this higher load early in the day will elevate the oil temperature sooner, and the higher temperature will linger once you slow down. This higher-load run will also help to burn off any soot accumulated on the previous run.

With a common rail ECU-controlled engine, you have far less risk, if any, of carbon buildup due to chronic light load conditions. Operating parameters will be adjusted to mitigate conditions that would lead to coking.

For both mechanical and common rail engines, we still must pay attention to oil temperature. It should be elevated enough to burn off any moisture. Monitor the temperature gauge until it reaches 212 degrees Fahrenheit.

If no gauge is fitted, then it’s a good idea to get in the habit of checking the temperature at the oil pan, or in an oil line leading into the oil cooler that’s fitted with an infrared pyrometer (also known as a touchless thermometer). Note that the oil in the oil pan may be slightly cooler than the temperatures reached in the engine.

If you find that you are not getting the oil hot enough, you will need to increase the load as described above. The focus on preventing engines from running too hot must be balanced with concern about running them too cool. With a little attention and some thoughtful running-time protocols, you can adjust conditions so that they are just right, resulting in a long-lived engine.

Just as a couch potato at home benefits from regular exercise, your engine needs a good workout on a regular basis.

This story originally appeared in the January/February 2022 issue of PassageMaker magazine.