Tech: D is for diesel, E is for engine, F is for fuel system

Rudolph Diesel invents the diesel engine.  (Artwork by Vela)
It seems a lot of people liked my Boating Electrical ABCs, which even got republished a couple of times, so here's a mechanical sequel.

Disclaimer: I am not a mechanic although I’ve taken a class or two. Mostly this is stuff I’ve just learned from tinkering and the "school of diesel knocks".

D is for Diesel

Diesel is the most common fuel used in marine applications, and for good reason. It’s a truly remarkable fuel. Firstly, diesel fuel is oily compared to petrol (gasoline). You may well have cursed this fact the last time you got some on your hands, and you’ve undoubtedly noticed that spilled diesel does not readily evaporate. Its oiliness means that diesel has built-in lubrication which prolongs engine life. Secondly, diesel is less flammable than petrol, due to its much higher flash point, i.e., the lowest temperature at which it can vaporize to form an ignitable mixture in air (74°C/165°F to be precise). And thirdly, diesel has a lower vapor pressure which essentially means it does not release as many fumes. This is especially advantageous in confined spaces, such as vessel engine bays, where the accumulation of a fuel-air mixture is potentially explosive.

The main thing you have to worry about with diesel is fuel contamination and there’s no prize for guessing that the most common contaminant is water. While a low concentration of water dissolved in diesel is not a big deal, high levels or undissolved “free” water are serious problems. At best, the presence of water reduces your engine’s energy output, much like water quells a flame. At worse, water can cause permanent engine damage, for example by destroying fuel injector nozzles. The presence of water causes another serious problem known as “diesel bug”, which are microbes that grow within fuel tanks feeding on the hydrocarbons in your diesel. The presence of these microbes not only degrades the quality of your fuel, thereby reducing power output, but creates biomass that gradually clogs up your fuel system. They even excrete acids that, over time, corrode anything metallic in your fuel system (a good reason to have plastic fuel tanks). Diesel bug is something that belongs in an Alien movie, not in your fuel system!

Diesel sludge
To avoid contamination:
  • Purchase diesel fuel from reputable suppliers, and as a precaution keep some spare fuel which is known to be good in a separate container.
  • Maintain your fuel tanks by cleaning them regularly.
  • Keep fuel tanks as full as possible to prevent water from condensing (and air getting sucked into fuel lines). Always, fill your tank before leaving your vessel for an extended period.
  • Add fuel biocide to minimize microbial growth.
  • Use high quality fuel filters.
  • Detect and fix any leaks.
The above photo shows some of the sludge I extracted from the bottom of Arriba’s fuel tank the last time I cleaned it. You don’t want this muck circulating through your engine.

E is for Engine

Volvo Penta D1-30 diesel engine: (1) freshwater cooling, (2) fuel filter, (3) primer pump, (4) intake silencer, (5) hot water outlet, (6) Electronic Vessel Control (EVC), (7) suspension (8) alternator, (9) drive belt, (10) engine block.
Did you know that diesel engines don’t use high-voltage sparks to ignite combustion? Unlike petrol and liquefied petroleum gas (LPG) engines, diesel engines are so-called “compression ignition” engines. Air inside the cylinders is compressed to high pressures, of the order of 40 bar (4 MPa or ~600 PSI), resulting in high temperatures, which in turn causes combustion.

Note: 1 bar (1000 mbar) is approximately atmospheric pressure at sea level, so that’s 40 atmospheres in your diesel cylinders. By comparison, water pressure increases by 1 bar for every 10 m (~33 ft) of water depth, so this is equivalent to a water depth of 400 m (~1300 ft).

As far as combustion engines go, the diesel cycle is an efficient process, typically converting ~45% of the fuel energy into mechanical energy, versus only ~30% for petrol engines. Further, diesels are not just fuel efficient when operating at their optimal load (i.e., typically your cruising speed); their fuel efficiency is relatively constant for all loads. In contrast, petrol engines are less efficient under part loads.

Note: Although diesel engine efficiency is relatively constant, you will of course still use less fuel at lower RPMs and more fuel at higher RPMs. It’s just that at lower loads you use even less fuel than you would compared to a less-efficient petrol engine. Diesel engines are good, not magical!

The absence of sparks is also beneficial in marine applications, since it eliminates a source of electromagnetic radiation (EMR) which can interfere with communications and navigation equipment (see my article on fluxgate compasses).

Further, the life of a diesel engine is generally about twice as long as that of a petrol engine. This is not just due to the superior lubrication properties of diesel fuel, but also due to the stronger engine parts used in their construction, necessary to withstand those higher pressures and temperatures.

Finally, as an added bonus, diesel engines generate less waste heat in the exhaust and emit minimal carbon monoxide (CO). As Rudolf Diesel, the inventor of the diesel engine might have said, “Wunderbar”.

On the cons side, diesel engines emit a noticeable rattling or clanking sound when running, which is called “knock”. The knocking is just a by-product of the high compressions and the fuel injection process. It diminishes once the engine warms up, but doesn’t vanish completely. The timing of the fuel being injected is critical to smooth diesel engine operation, and incorrect timing will result in severe knocking.

Other than noise, the main downside of diesel engines is the possibility of runaway failure. Since no ignition is required for combustion, diesel engines only regulate their speed by controlling the fuel supply. Therefore if you’re unlucky enough to have fuel pump break down in the "on" position, the supply of fuel will continue unabated and the engine will keep running until something breaks.

Tip: Every skipper really needs to be familiar with the quickest way to quickly shut down a runaway engine. Shutting off the air intake is one such method. Shooting a CO2 fire extinguisher into the air intake is another method.

Diagnosing diesel engine problems is beyond the scope of this article. There are long books devoted to this subject! Nevertheless, here are few tips I’ve learned to keep engine(s) happy.
  • Be gentle. Let your engine warm up at low speed and low load so that it reaches normal operating temperature before using full power.
  • Don’t idle your engine only to turn it off too soon before it has had a chance to warm up. Doing this means the engine never gets hot enough to burn off any water, so you’ll just end up with more water condensation next time. Conversely, periodically idling your engine for just 2 or 3 minutes to circulate oil is better than not running it at all for 6 months.
  • Once the engine has warmed up, listen carefully for variations in the engine power. For a given speed (RPM), the engine should run and sound steady. If the engine starts surging or “hunting” and fluctuating in power, you most likely have contaminants or an air leak.
    Note: Abnormally loud knocking indicates more serious problems, such as faulty injectors. Unless you like taking apart engines, it’s time to call the mechanic.
  • Keep the engine bay clean by wiping it down regularly with a light solvent. If your engine runs dirty, that grime will work its way into everything else nearby, i.e., pumps, macerators, switches, alternators, etc. In particular, your alternator will suffer if its windings get coated in grime. A very dirty engine bay could signal a small leak that may get worse over time. Also, touch up any spots where the paint has worn off your engine to prevent rust.
  • Change the engine oil twice as frequently as your engine manufacturer suggests. If they recommend changing the oil every so 200 hours, do it every 100 hours. Oil is your engine’s lifeblood and clean oil is the single biggest factor that will prolong its life. (A mechanic gave me this tip.)
    Tip: If there is only one thing you do to care for your engine, it should be to regularly change the oil.
  • Regularly check all fluid levels (engine oil, gear lubricant, coolant, etc.). A sudden drop in any level is a giveaway that something is wrong. For example, I once experienced a sudden loss of coolant due to a leak in the connection to my hot water tank (which recirculates the waste heat from the heat exchanger).
  • Finally, and most importantly, familiarize yourself with your engine operator’s manual and also treat yourself to a copy of the workshop manual, i.e., what marine engine mechanics refer to when servicing engines. (You can often find soft copies online). I also recommend the following two books: “Troubleshooting Marine Diesels” by Peter Compton.  and “Marine Diesel Engines” by Nigel Calder. Nigel Calder’s “Boatowner's Mechanical and Electrical Manual” is also a useful, albeit hefty, addition to the boat library. 
My library of engine books

F is for Fuel system

The fuel system is the catch-all term for everything between the fuel tank and the engine. First there are various hoses, namely a supply hose from the tank and a return hose going back to the tank (since not all the of the fuel is combusted). A low-pressure fuel pump, called the “lift pump” or "feed pump", draws fuel out of the tank and a second, high-pressure “injection pump”, pressurizes the fuel injectors. In electronically-controlled engines the injection pump pressurizes a “common rail” at pressures as high as 3000 bar (300 MPa or 45,000 PSI) in turn supplying the injectors, which are opened and closed by an “engine control unit” (ECU). In mechanical engines, such as Arriba's Volvo Penta D1-30, the injectors are connected to the engine via a cam shaft which drives plungers dispensing fuel to the injectors.

Note: You need to take extreme care when working on injectors, since escaping fuel at those pressures can penetrate skin and be deadly! Some things are best left to mechanics.

If fuel never got contaminated this would be the end of it. Alas, in the real world, fuel does get contaminated and water will almost certainly get into your tank. Filters to the rescue! The first filter, usually referred to as the “pre-filter” or “separator”, separates out the water and the second filter, separates out the undesirable particles. Your tank may also have a “tank filter” located inside the tank, in which case this is your first defense in eliminating fuel contaminants.

Fuel system for Volvo Penta D1-30:  (1) fuel tank, (2) pre-filter, (3) lift pump, (4) fuel filter,
(5) hand pump, (6) injection pump and (7) injectors, one per cylinder.
The most common problem you’re likely to encounter with your fuel system, is not a pump or injector failure, but a simple leak (followed by a clogged filter). (FTR, it was an annoying leak that prompted me to start writing on this subject.) Leaks suck, literally! They don't just spill fuel; they suck in copious quantities of air due to the partial vacuum created by the Venturi effect. Too much air in the fuel is as bad as too little air, since it prevents the diesel engine from developing full power. If the air is not “bled out” (vented) eventually the engine will conk out completely and then refuse to start. Some modern diesel engines are “self bleeding”, but many are not. Look for the instructions in your engine manual on bleeding, venting, priming or words to that effect. Basically, the process just involves pumping out the oxygenated fuel until the bubbles stop and clean fuel starts to flow out smoothly (i.e., without bubbles).

Tip: One day you will get air in your fuel system, so knowing how to vent your engine could well be a life saver. It only takes a few minutes once you know how.

Sadly, the worst occurrence of air in your diesel fuel system has absolutely nothing to do with leaks. It is due to operator error when the skipper is careless enough to run out of fuel, resulting in “air lock”. Air bubbles can get trapped in the injectors, which prevents the flow of fuel, etc. I made this mistake once and had to top-up my fuel tank at sea from a jerry can, then re-prime the system - all while single handed. I vow never to run out of fuel again!

The source of leaks are varied and many, but here are some of the common locations:
  1. Fuel injector return hoses
  2. Fuel tank hoses and clamps (both at the engine and fuel tank)
  3. Seals on fuel pre-filter/separator
  4. Seals on main fuel filter
  5. Hand primer (vent/bleed) pump
  6. Fuel injector lines
My high-tech leak detection kit entails a roll of toilet paper and some masking tape. To track down my leak I carefully cleaned and dried all the hoses and wrapped TP around each joint. I eventually tracked my leak down to the gasket on the hand primer pump, in the center of the photo (#5 on my list).

Leak detection with toilet paper.
In addition to external leaks, fuel injector nozzles can also leak internally into the engine, but don’t ask me how to diagnose such a problem!

Here are a few tips for keeping your fuel system happy:
  • Drain the accumulated water out of the pre-filter/separator monthly, to minimize the chance of water circulating through your engine.
  • Inspect the clarity of the fuel drained from the pre-filter/separator (or eyeball it directly if you have a plastic bowl). Fuel should be very clear when held up to light. Hazy fuel is invariably contaminated fuel, mostly likely due to diesel bug. Kill the bug with a biocide, and then remove it’s dead biomass from your fuel tank. The tedious, manual way is to drain your tank completely then scrub it clean, assuming you have good access. (I have have three 20-litre jerry cans I use for this purpose).
  • Alternatively, consider implementing a “fuel polishing” system, described here. (Caveat: I haven’t done this, but people seem to swear by it.) That said, no amount of “polishing” will fix a fuel tank that is badly encased in bugs, dead or alive. You’ll have to clean your tank manually.
  • Never use paper products (such as paper towel) to clean your tank! The paper fibers will disintegrate and enter your fuel system, clogging filters, or worse, damaging injectors.
  • For the same reason, never use teflon tape on seals. Instead, use a high-temperature thread sealant.
  • Change fuel filters at least yearly. Trapped contaminants eventually clog the filter and restrict fuel flow, in turn reducing engine power.
  • Check the the tank filter too (if you have one). A clogged filter inside the tank will cause as much loss of power as a clogged filter elsewhere.
Diesel engines are the workhorses of the oceans. Take proper care of your engine, and you can depend upon it. Neglect engine maintenance at your peril though, as Murphy’s Law has a habit of striking when you least expect it, and for some reason seems to pick on engines!