George B. Bellwoar

As mentioned in our last newsletter, there are numerous benefits to adding a diurnal emissions control fuel system to gasoline-powered boats. But how do these systems work?

Today’s two most popular diurnal emissions control systems are pressurized systems and canister (often called atmospheric) systems. Pressurized systems prevent outward venting until internal system pressure exceeds roughly 1.25 psi. This usually happens only with significant temperature swings.

Canister systems operate at atmospheric pressure. Certified carbon canisters in the vent line capture hydrocarbons from venting vapors during the day and expel them back into the tank at night when cool air causes the system to breathe in. Most automobiles use a complex version of the marine canister system.

Both systems need airspace (or ullage) at the top of the tank to ensure that a gas (air or vapor) will be vented, not liquid fuel. Both systems can (and usually do) use the same approach to creating that ullage. Typically, they use three different mechanical control valves.

• Fill Limit Valves (FLVV) – These valves control the ullage volume. A large 5/8″ (16mm) vent allows a high volume of vapor to pass through during refueling.
• Tank Vent Valves (TVV) – These are also called Rollover Valves (ROV) or Grade Valves (GRV), depending on the manufacturer. They have a much smaller 5/16″ (8mm) vent size and control venting anytime the FLVV is closed. Diurnal venting often passes through TVVs.
• Inlet Check Valves (ICV) – ICVs are usually mounted in the fill hose and prevent fuel from coming out of the fuel fill if the tank “burps” immediately after the FLVV closes. ICVs prevent wellback.

Systems typically have one FLVV, one or two TVVs, and one ICV. Variations exist depending on the size and shape of the tank and the overall system design. During refueling, incoming fuel hits the ICV and opens it. An FLVV vents the high volume of refueling vapor until the fuel level in the tank reaches and closes it. Tank pressure then spikes as fuel comes in, but no air leaves. This closes the ICV, fuel backs up the fill hose, and shuts off the pump. Then, the TVVs slowly vent the tank pressure. As it drops, the ICV opens, and the fuel in the fill hose drains into the tank. When someone rounds off the cost by clicking the pump several times, they repeatedly refill and empty the fill hose into the tank. If someone does this long enough, they will reduce the airspace and eventually defeat the system. Hence the warning on cars and pumps not to overfill.

The fuel cap is another critical component in these systems. It seals when closed and “clicks” like your car’s. Caps used in pressurized systems have built-in pressure and vacuum relief valves, usually the only system vents. Caps used in canister systems often have the same built-in pressure and vacuum relief valves. But these are used as a system failsafe or for additional makeup air when high horsepower engines run flat out. Actual venting in canister systems is often via a conventional vent on the side of the hull.

In simple terms, that is how these systems work.

While these are the two most common approaches, other unique ways to control diurnal emissions are available. Enviro-Fil systems use a venturi effect rather than a pressure spike to shut off the pump. And Perko also offers a system to control emissions without any tank-mounted valves. There are options to fit every application. System integrators like Perko and Attwood help fuel tank manufacturers and boat builders to determine and then design the system that best fits the need of every boat model.

Look for our next newsletter, where we’ll talk about matching system design with system goals like safety, installed cost, range, and ease of refueling. We’ll also discuss the cost of these systems and the advantages and disadvantages of the most common approaches.

George B. Bellwoar, GMBA United States
Email: george.bellwoar@gmba.blue
Tel:: +1 954 646 5920

Disclaimer: Global Marine Business Advisors is a registered legal entity and is a network of independent marine industry advisors. In all articles the opinions expressed are those of the author and does not necessarily reflect those of GMBA.

When considering emissions control in the marine industry, many look at engines and manufacturing processes as the culprits. For sure, both contribute. But they are not the only sources of emissions. For example, spilled gasoline and diesel fuel both pollute our water. And evaporating gasoline adds to air pollution. And there is another source of pollution from boats that many don’t even realize exists – diurnal emissions.

The diurnal cycle describes the daily temperature swing from nighttime to daytime. As the earth rotates, the sun warms the parts it faces during the day. Then those parts cool at night. Materials expand when heated and contract when cooled. Gasoline is no different. It expands quite a bit with temperature changes. It can grow nearly 1% for every 15⁰ F (or 1% for every 8⁰ C.). So, a tank with 200 liters at 6⁰C. could have 210 liters at 46⁰C., a reachable temperature in a boat stored in blue shrink wrap.

Unlike water, a molecule, gasoline is a complex liquid solution of many different (mostly hydrocarbon) molecules. Several of these molecules evaporate around room temperature. It is one of the reasons that gasoline gets “stale” over time. So, the top of a half-filled tank contains hydrocarbon vapor, not the air you’d want to breathe.

We have diurnal emissions when the evaporation of hydrocarbons combines with the diurnal cycle that expands and contracts fuel and pushes vapors out of the tank daily. Gasoline-powered boats stored outside emit hydrocarbons from their tank through the vent and into the atmosphere daily.

Since 2010, the United States has controlled diurnal emissions from boats by law. It has been the only country to do so. There is “talk” in Europe and Australia, but regulations are probably 3 to 5 years away.

Therein lies an opportunity for boat builders. Many talk the green talk. However, when it comes to diurnal emissions, only a few European builders walk the green walk.

Automobile manufacturers worldwide added diurnal emission control nearly 50 years ago. Boats sold in America have had similar features for over a decade. This technology is proven. It even offers benefits beyond diurnal emission control, some of which apply to diesel-powered boats. For example, boats using these systems force automatic pump shutoff – just like in cars. So gone are the days of watching for a squirt of fuel to know when your boat is full. Also gone are the days of fuel leaking out of your vent on a hot day. These systems make filling boats easy and eliminate premature shutoffs. Ensuring ullage (airspace) in the tank allows room for fuel expansion. The release of harmful vapors into the atmosphere gets controlled while the evaporation of expensive gasoline is limited.

Diesel systems differ from gasoline systems because, unlike gasoline, diesel fuel doesn’t readily evaporate once spilled. So, spillage is the problem, not diurnal or evaporative emissions. When spilled, diesel fuel floats around until it eventually sticks to something. Therefore, avoiding diesel spills is especially important. And doing so is surprisingly inexpensive. Since we’re not worried about evaporative emissions, diesel systems only need to create airspace and shut off fuel pumps to prevent spills, which can often be done for less than 100 Euros on smaller diesel boats under about 20 meters.

In most cases, even complex systems are affordable. In all cases, the benefits are great. And boat builders, system integrators, and designers can customize these systems based on varying goals, such as tighter emission control, ease of installation, price, safety, space utilization, and even range.

In our next article on diurnal emissions, we’ll explain how US builders do this, describe the different types of systems available, and then discuss the pros and cons of each approach.

George B. Bellwoar, GMBA United States
Email: george.bellwoar@gmba.blue
Tel:: +1 954 646 5920

Disclaimer: Global Marine Business Advisors is a registered legal entity and is a network of independent marine industry advisors. In all articles the opinions expressed are those of the author and does not necessarily reflect those of GMBA.