Clean Fuel, Happy Engine

Taking care of a boat's fuel system takes care of a boat's diesel engine.

Advancements in diesel engine design and function may not be taking place as rapidly as, say, in the marine electronics realm, but make no mistake, they’re happening — and consumers will reap the benefi ts. Anyone who hasn’t read up on marine diesels recently might be surprised by what’s happening. We have the basics here.

Better Power-to-Weight Ratios

Modern engines are significantly lighter than their predecessors, producing twice the power at half the weight of older comparable engines, in some cases. Older engines require up to six pounds per horsepower, but newer models can produce one horsepower for less than two pounds. Lighter, more powerful engines with a reduced footprint will allow older boats with limited engine-room space to be repowered with higher horsepower units. This combination of less weight and more horsepower often translates to greater boat speeds.

Even better, the reduction in weight doesn’t sacrifice performance or durability. Just because that new engine is lighter doesn’t mean it is a lightweight. Design innovations coupled with the greater use of advanced alloys allow manufacturers to reduce weight without sacrificing expected engine service life or reliability.

The use of advanced technology such as constant computer monitoring means problems are caught well before the point of catastrophic failure, reducing repair costs and downtime while increasing service life.

One caution when comparing engine weights, however: Be sure to verify whether the published weight does or does not include the reduction gear. Some manufacturers include them but other manufacturers don’t.

MTU 10V2000 M96

MTU 10V2000 M96

Efficient, Greener Tech

Advances in engines’ fuel delivery — specifically electronically controlled fuel systems — are a boon to both the environment and an owner’s wallet. Replacing mechanical injection pumps with single-rail fuel delivery — where the injector is computer controlled — allows digitally controlled engines to make thousands of instantaneous calculations and provide the best performance possible. Improved fuel injection control makes multiple fuel injections per stroke possible, giving a new engine the ability to efficiently wring the most benefit from every drop of fuel.

Another benefit of all this improved fuel injector control? The higher response rate of electronic injectors and the fuel pulse shaping it provide a quieter running engine — as much as an 80 percent noise reduction at idle in some engines.

Fuel systems that include large primary filters and smaller secondary filters will deliver a solid first line of defense for engines. Clear bowls make it simpler to monitor the fuel’s cleanliness.

What’s not to love about a quieter engine that produces less smoke and delivers better fuel economy while extending a boat’s cruising range?

Fuel & More

Speaking of engine efficiency and fuel economy, according to national repair statistics, nine out of 10 problems owners are likely to encounter with new diesel engines will be the result of contaminated fuel. Don’t fret over that too much, though. Here’s how to keep a boat’s fuel system in top condition and its diesel engine from becoming a statistic.

One of the major differences between gasoline and diesel engines is the way they use fuel. Gasoline is simply a fuel that’s burned to produce heat and power, but diesel fuel additionally acts as a lubricant, which is why it feels oily. Diesel engines circulate more fuel than is needed to produce power, and this “extra” fuel not only helps lubricate the engine but also carries away excess heat.

The key to keeping a diesel engine happy is clean fuel. The reason for all of this clean fuel hubbub has to do with diesel engines’ fuel injectors. As I mentioned earlier, these precision-tuned components deliver a precise, ultra-fine mist into the combustion chamber. They don’t like contaminants, and even microscopic specks of dirt or water can wreak havoc on the combustion process — as well as on the injectors and the kid’s college fund.

Racor Turbine Fuel Filter

Racor Turbine Fuel Filter

Loving Care

The 10 commandments of diesel fuel care go something like this:

  • Buy fuel from a reliable source.
  • Filter fuel as it comes aboard.
  • Add fuel stabilizer and biocides, particularly in the case of long-term storage.
  • Keep water out!
  • Install a robust fuel filtration system with dual primary filters.
  • Install a vacuum gauge to monitor fuel filter performance.
  • Regularly check filter bowls for signs of contamination.
  • Change all fuel filters/elements on a regular basis as per the engine manufacturer’s recommendations.
  • Keep fuel tanks clean, and clean dirty tanks immediately.
  • Install an onboard fuel polishing system.

First Line of Defense

A typical diesel fuel system includes a large primary filter mounted on a bulkhead near the engine and a smaller secondary filter mounted on the engine itself. The primary filter does the lion’s share of fuel filtration, while the finer mesh secondary unit does the cleanup work by filtering out any microscopic particles of grit and water that make it past the primary. Many diesels will also have a second primary filter plumbed into their fuel system, allowing owners to change a clogged filter while keeping the engine running.

Primary filters should have a clear sediment bowl that allows owners to visually check for water or sediment, which should be part of their daily routine while cruising. Fuel filters should be changed when dirty, but definitely prior to affecting engine performance. Owners can remain vigilant by replacing filters at regular intervals — X amount of hours or annually, for example — but a more accurate way to monitor filter performance is to install a vacuum gauge at the primary filter. It provides a visual representation of fuel flow restriction through the primary filter or, conversely, how hard it is for the engine to “suck” fuel through it. The higher the gauge pressure, the more clogged the filter is, indicating the need for replacement.

To help keep bad fuel from entering the tank in the first place, use a multistage fuel filter funnel during fueling. Popular examples of these include the Racor Fuel Filter Funnel ( or Mr. Funnel (

One final note on fueling. Even with a fancy new filter funnel and other such precautions, the best option is to always purchase the cleanest fuel possible. Choose marinas that have a high rate of fuel sales and, thus, fuel turnover. One of the best marinas I ever saw in this regard was located beside a major highway and also served as a truck stop.

If in doubt about the cleanliness of the fuel, pump some into a clean glass jar before fueling and let it sit for a few minutes, after which any water or dirt present will settle to the bottom. If either is present, go elsewhere for fuel.

Whether it’s a small engine space or a more expansive room such as on the Monte Carlo Yachts 65, a primary fuel filter, often mounted on a bulkhead near the engine, is a vital component of a diesel fuel system.

More to the Story

Buying clean fuel, however, is only half the battle. Two of a boat owner’s biggest enemies in the battle for clean fuel are water contamination — once the fuel is on board and during long-term storage — and sulfur. Diesels use pressure to generate combustion, and when water enters the engine it turns to steam, which can literally blow injectors to pieces. Water also mixes with the sulfur in fuel to create sulfuric acid — sort of like the blood in the critters in the “Alien” movies — which can cause internal engine parts to corrode.

So how does water get into the fuel tank? One way is through the tank’s vent system. Unlike the closed fuel system in a car, a boat’s tank is vented, creating an open-to-the-environment system that lets moisture in, where it can condense on the inside walls of the tank due to daily heating and cooling cycles. The more air in the tank, the more moisture and potential condensation. To combat this, one recommendation is to keep the fuel tank full (up to 95 percent), particularly during long-term storage.

Another common point of entry is the fuel tank’s deck fill, usually due to damaged or missing O-rings. Fuel fills located on sidedecks are especially vulnerable in this regard, particularly on sailboats, since they can ship a lot of water during rough passages, heavy rains or even during washdowns.

Volvo Penta D3-220

To the Web

Find out more about the latest marine diesel engines from the manufacturers on the forefront of technology and development.

Long-Term Commitment

After water enters a fuel tank, it eventually separates and settles to the bottom — a common problem with fuel stored for a long time. That’s when it gets buggy. Microbes thrive in such water, and their only goal in life is to eat and multiply. Sure, biocides — usually introduced after the fact — will kill the little SOBs, but as any good hit man knows, killing is the easy part; the problem is disposing of the bodies. In the case of dead microbes, they’ll lie in wait at the bottom of a tank until a rough passage and then rise up, zombie-like, to clog filters and wreak general vengeance on the fuel system or, worse, the engine.

The best strategy, particularly for longterm storage, is to treat clean fuel with stabilizers and biocides before a bug issue occurs and prevent water from entering the tank in the first place. No water, no critters.