4wings.com Hovercraft development 47751 Michigan ave., Port Isabel - 78578 - Texas, US Tel.: (956)943 5150

4wings.com

Hovercraft development
47751 Michigan ave. , Port Isabel, TX, 78578
Tel:(956) 943-5150












Considerations for fuel system on high speed Hovercraft

This file is provided free for educational purposes with the assumption that the reader is fully aware of the risks involved. The author assumes no responsibility or liability for its use or consequences of its use by others. 

Marine fuel system considerations
 
 

Fuel inlet : 

Fuel inlet need to be located on the exterior surface of your craft in a manner that no spilled fuel can find it's way into the bilge of the craft.
Most likely you will use a "plastic fuel inlet with vent" These units are very lightweight and do not require any ground cable as the metal counterparts would require.
 

Fuel strainer:

It would be wise to fill your fuel into the tank through a fuel strainer. These units come sometimes with fuel tank's but are more or less just a fine filter. Some fuel funnels have strainer fix installed. Most of your fuel contamination will come from your local gas station. The fuel strainer will not allow you to fill the tank as quick as without - but might avoid major problems down the fuel system line and major damage to the following components.
 

Fuel tank :

A key part of any fuel system is the fuel container itself.  The debate is whether to use a modified  USCG approved fuel tank or install a racing fuel cell.

There are several benefits for retaining the stock fuel tank in a high horsepower Marine craft.  It has a larger capacity than most fuel cells, already has a mounting location and hardware, has provisions for filling from outside the craft, has a cap that both vents and seals and is already on/in the craft.

The drawbacks of using a common USCG approved fuel tank are more numerous but less obvious.  The stock pick-up / pump assembly is restrictive, requiring complete replacement with a fabricated assembly.  When using a stock tank with fabricated pickup, unless the fuel level in the tank is full or higher, the internal well, which the stock pump draws from, is far too small and poorly supplied with fuel from the rest of the tank.  Faced with the demand of a large pump, drawing through a fabricated pickup, it has no chance of refilling fast enough to support WOT full engine load.  Under low demand, e.g. cruise type conditions, the large volume of fuel delivered to the carburetor or rails is unused and returned.  The same fuel, picks up heat from the pump and the rails, is constantly recycled to and from this tank, which will rapidly increase fuel temperature.  Common problems associated with stock fuel tanks and fabricated pickups are pump cavitation, vapor lock, varying fuel pressure, exaggerated pump wear and lean conditions during both low and high loads. Note: Unlike a carbureted engine, any loss of fuel supply at the in-tank- pickup will immediately result in a loss of fuel volume and pressure at the EFI injector resulting in lean conditions and engine damage.

Most of us will be stuck with a  stock style fuel tank since they need to be USCG approved to qualify for a marine craft , you look at a common layout of 3/8" pick up line with anti syphon valve as "Pick up". This pick up is restraining your fuel flow at the same source where it should flow free. Your tank will most likely have a   1 1/2" fill and 5/8 " vent tube  molded into the  tank. Here we are still missing one more nozzle for the cozy/warm return line.
Until the tank manufacturers  have changed their tank design to provide bigger diameter pick up lines and a return line nozzles, we can only modify it to fit existing needs. We can use the 5/8" vent tube and modify it to the pick up line, lets not forget to  install a anti syphon valve before we go to the filter / pump. Use the 3/8" pick up line as vent ( make sure to get rid of the pick-up tube molded into the tank) and "T" into the 1 1/2"  fill line your return line at the closest point to the tank. As the pump will start to pick up fuel from the tank, the fuel volume will decrease. Any excess fuel from your carburetor or EFI injectors will be returned into the tank and the warmer fuel will mix with the fuel in the tank. Do not "T" your return line just before the fuel pump since that will send high amounts of fuel continuously through the fuel pump to heat up more till it will reach critical temperature.  Unfortunately with this modification,  you have changed the once USCG compliant tank into your own piece of art.. Even though this piece of art  might do the job but you are not USCG compliant anymore. 
With this set up the primary problem of heat soak is minimized.  However, this is still a compromise at best, requiring fuel levels be maintained above for normal low engine load, above to for  racing .  In all serious racing applications the correct fuel cell is highly recommended. 
 

Fuel Intake filter:

Or intake filter - Most people regard fuel filter as the first line of defense against contaminated fuel. Personally, I believe it is the last line of defense which often  produces more problems than it solves. This part of the fuel system file is regarding non diesel application which have a  bigger set of problems.
Before you actually fill fuel in your tank, make sure the tank and the fuel  is clean. Use a strainer every time to fill the tank and close fill cap after filling.

A good recommendation from  Aeromotive fuel system is that the  filtration media to be used on the inlet side of a fuel pump may be no smaller than 100 micron and must have an element surface area of 60 square inches or more.  Any filter element not meeting these criteria may fail to flow the full volume of a high capacity fuel  pump being used, resulting in cavitation at the pump inlet. Most high capacity fuel pumps are extremely efficient by design, allowing them to create high pressure on the outlet and high vacuum on the inlet side, if restricted.  Cavitation can be to a pump like detonation is to an engine and occurs when the liquid being pumped reaches a temperature where it boils and starts to vaporize.  The temperature at which any liquid boils varies with pressure.  Recall that water in a radiator is purposely pressurized to raise the boiling point.  When was the last time your high pressure EFI system vapor locked?  Keep in mind, as a pump pushes it has to pull.  When a pump has to pull too hard acquiring fuel, a vacuum or low-pressure area develops at the inlet.  The better and more efficient the pump is, the lower inlet pressure will fall.  The boiling point of any liquid fuel in this low-pressure zone falls as well.  With a highly efficient pump, inlet pressure can get so low that fuel will boil and the pump will cavitate at normal operating temperatures. Today's ultra-high output engines require equally high efficiency fuel pumps.  Failure to install them properly can be costly in two ways:  First, during cavitation the engine may experience a momentary lean condition (loss of liquid fuel pressure and volume).  Second, excess heat and friction will build in the pump, causing damage and eventual failure.  If you feed your pump properly it will feed your beast for years to come!  Review your installation and make sure the pump is mounted where gravity will help push fuel to the inlet, use the correct size AN line between the tank and the pump and install filters that flow the necessary volume freely. In some marine applications you will find a whole set of filters before the actual pump, make sure your filters are cleaned and cartridges replaced on a regular basis. 

 

Fuel pump :

The performance of your whole fuel system depends upon the fuel pump. Selecting the right fuel pump is of the out most importance to your high speed water craft's performance.
The critical factors that effect fuel pump selection are numerous.  In the past, fuel pump manufacturers have rated their offerings based on gallons-per-hour, free-flow (no test pressure), and with no reference to test voltage.  In the real world, this gives no indication of the horsepower that could be supported by such a pump.
The key variables that determine which fuel pump is suitable for a particular engine combination are:

  • Engine flywheel horsepower.
  • Engine fuel efficiency, commonly referred to as BSFC or Brake Specific Fuel Consumption.
  • Maximum fuel system pressure and the pump's flow volume at that pressure.
  • Available voltage at the pump under engine load and the pump's flow volume at that voltage.
The first step is to establish how much horsepower will be produced and the amount of fuel required to support it.  To be safe, start by estimating HP on the high side and efficiency or BSFC on the low side.  A typical gasoline engine will use less than 1lb of fuel to make 1 HP for 1 hour, so expect the BSFC number to be less than 1.  Different engine combinations, power adders, even fuel octane ratings and tuning approaches will have a profound impact on BSFC.  Consider this carefully when choosing a fuel pump. 

You may use the following information as a guideline, however these are simply observations.  The best, and our recommended, method of establishing actual BSFC is through proper flywheel dyno testing.

    • Naturally aspirated engines are normally most efficient with a BSFC between .4 and .5 lbs/hp/hr. 
    • Nitrous combinations use a little extra fuel and often develop a BSFC from .5 to .6 lbs/hp/hr.
    • Forced induction engines are usually least efficient and BSFC ranges from .6 to .75 lbs/hp/hr.
Using 650 HP, lets figure the fuel requirement for the most vs. the least efficient engine combination. 
  •  650 HP multiplied by a .4 BSFC equals 260 lbs of gasoline. 
  • 650 HP multiplied by a .75 BSFC equals 487 lbs of gasoline.


As you can see, the amount of fuel required to support two different engines, each making the identical amount of HP but with very different fuel efficiencies, virtually doubles the volume of fuel required! 

Note: It is equally important to consider BSFC when determining minimum injector size.  To calculate, divide the lbs of gasoline required by the number of injectors used.  If you are estimating, it pays to be safe.  Many engine builders will add a percentage to total fuel pump volume for safety and then divide the minimum injector by .8 in order to target about 80% injector duty cycle.  This allows consistent injector performance, cooler operation for enhanced durability and leaves about 10% for unexpected power.

For example:

  • 650HPx.4 = 260lbs.   260lbs/8 injectors=33lbs/hr.  33/.8=41lb/hr injector @ 80% duty cycle.
  • 650HPx.75=487lbs.   487lbs/8 injectors=61lbs/hr.  61/.8=76lbs/hr injector @ 80% duty cycle.


It is imperative to consult with an experienced engine builder when estimating HP and making these calculations.  There's a lot at stake and errors can result in serious harm to the engine and those around it.

Determining the fuel volume necessary for a particular engine is the first step in selecting a fuel pump.  If the combination is naturally aspirated, does not use rising fuel system pressure and has a correctly sized alternator in good working condition it may be OK to stop here.  If not, there's still more to consider.

The second step is to establish what the base fuel pressure will be and if, as with forced induction or certain "dry nitrous" kits, pressure will be required to change with engine load. How does fuel pressure affect pump delivery?  You can bet that as system pressure goes up the pump’ volume will go down. 

To illustrate this, take one of the most popular and efficient EFI pumps on the market, Aeromotive’ A-1000 part #11101.  Lets examine various pressures to demonstrate the effect this has on flow volume:

  • Carbureted, Nat Aspirated, 9psi and 13.5v, volume 791lbs/hr.  1,582 HP @ .5 BSFC. 
  • EFI, Nat. Aspirated 43.5psi and 13.5v, volume 614lbs/hr.  1,228 HP @ .5 BSFC.
  • 20psi boost/1:1 Regulator, intercooler, 60psi and 13.5v, volume 529lbs/hr.  881 HP @ .6 BSFC
  • 10psi boost/4:1 FMU, intercooler, 80psi and 13.5v, volume 426lbs/hr.  710 HP @ .6 BSFC
  • 6psi boost/8:1 FMU, intercooler, 91psi and 13.5v, volume 370lbs/hr.  616 HP @ .6 BSFC


Measuring a high efficiency fuel pump such as "HP Marine USCG Fuel Pump P/N 11108" , from 9psi to over 90psi, flow volume is reduced a total of 53%.  Comparing volume at 60psi for a high boost kit with correct injectors to 90psi for a low boost application, with small injectors and an FMU, volume is reduced by 28%. 

Clearly the effect of rising fuel pressure has significant impact on flow volume.  What is not shown (and rarely published) is the devastating impact this has on less efficient, traditional pumping mechanisms.  It is obvious that eliminating unnecessary fuel pressure rise, e.g. removing an FMU and installing the correct injector, increases flow, maximizing the HP potential of any fuel system.

This brings us to our third fuel pump performance factor; voltage supply as measured at the fuel pump terminals.  Voltage to an electric motor is like fuel pressure to an injector, more pressure in equals more volume out.  Higher voltage at the pump terminals increases motor torque, resulting in more rpm and an increased flow volume for a given pressure.  To illustrate this, lets use again the above Aeromotive marine fuel pump at 80psi will see a 40% increase in volume when voltage is increased from 12v to 13.5v.  This factor is often overlooked and can make or brake pump performance, especially at high pressures.  The key here is to figure flow at voltage if an alternator is used or not.  As Hovercraft can fly where no other transportation means can go - you know you are the only one out there.
Just imagine if your alternator fails and and on your return trip voltage will drop. If the voltage of your slowly discharging battery reaches the critical aspect where your pump can't build sufficient pressure to supply the engine with fuel. I guess you are stuck for good. In case of a recreational craft you can make a nice adventure out of it, in case you  run a charter / taxi service you will need good legal representation.
 

Dynamic vs. Static fuel systems: EFI efficiency for carburetors.

CARBURETORS STILL RULE in many forms of racing and on many cruising machines.  Those who choose (or are required) to run a carburetor are turning to expensive, custom-built models for better performance.  Often the custom shop will advise a fuel pump upgrade, usually suggesting a pump rated to flow as much as 4-6 times the amount necessary to support the engine’ horsepower.  Normally the quick explanation for this is the need to overcome acceleration G-forces. 
Hold in mind you can spin your Hovercraft like a donut on water or hard surfaces. That you can do this is your pure fun, pleasure  and entertainment - just under this condition your standard fuel pump has to work overtime. Your fuel tank is most likely located as close as possible to the CoG of your craft, way down in the bilge area. Your fuel pump is located aft at the engine compartment. As long as the tank is aft of the Cog centrifugal forces are no major aspect, once the fuel tank is forward of the CoG centrifugal forces will first help the pump - before she has to overcome the centrifugal forces which force the fuel away from the CoG.
The least aspect you want to worry while you enjoy the donut flight is a dying engine because of low fuel pressure. Which would eliminate any control over your craft and most likely a bit wet plow in as you loose lift air.

The fuel systems first priority is to keep the floats from running low enough to uncover the main jet, running the engine out of fuel.  Traditional, static systems do a fair job of this.  The second, more difficult priority is keeping the fuel level optimum in the bowl.  It may not seem significant but the weight of fuel above the main jet does impact fuel flow through it, and therefore the air/fuel ratio of the engine under load.  The sophisticated carburetor racer knows the float bowl must always be as full as possible.  This is critical if engine tune is to be held across the rpm band, achieving peak performance throughout the race. An area of special focus for racing  crafts  is the time before getting over hump speed following launch.  Here the typical static fuel system struggles.  The fuel is standing in the line, barely moving and to make matters worse, the static style regulator places the check valve between the fuel pump and carburetor, restricting fuel flow across the board.  To combat this, fuel pressure in a static system is always held higher from the pump to the regulator (12-60psi) than it is from the regulator to the carburetor (3-7psi).  This higher line pressure is necessary for two reasons, one to start flow against G-force and two, to push fuel through the restrictive regulator valve.  A return style regulator places the inlet and outlet ports above the check valve with only the return volume having to flow through the restriction.  As a result, the pressure from the pump to the regulator is the same as from the regulator to the carburetor (3-7psi), allowing the pump to speed up, increasing volume significantly, and supplying full output directly to the float bowls at all times. 

The benefits of a dynamic, return style fuel system are numerous, including longer pump life, a marked increase in pump to horsepower ratings (allowing smaller, lighter pumps to fuel more HP), even quieter pump operation is common.  Ultimately, combining a high quality marine fuel  pump and return regulator into a dynamic fuel system significantly improves average float level, fueling the bowls more quickly and consistently.  The bottom line is the finish line and getting there first with a more constant air/fuel ratio across the rpm band and more predictable power all the way down. 

 

Outlet filer:

One more time we try to clean the fuel, preventing debris from damaging your carburetor or fuel injectors, with minimum flow restriction..
The main aspect is to filter extreme small contamination out of the fuel. Some filter units can be connected direct to your pump while others are mounted to your crafts main structure. Fuel filter which are kind of free floating in the air are a big no.
Please hold in mind that USCG regulations require that all components of your fuel system ( pump, filter, .... ) are fix attached to your craft. so free floating filter units are not compliant. If the filter does not provide any brackets you need to fabricate them "in house". Outlet filter cartridges have most of the time a 10-micron fabric element of the throw away type.

Regulator:

A regulator is required in both configurations - carburated engines as well as EFI ...unfortunately here you have to research on your own which unit you might need since every application has its own pressure requirements which need to be followed for optimum performance. Just try to make your homework before you call any technical support so they can really help you. Know your system, engine, if carburated or EFI, which pressure you require .....

Fuel lines :

USCG requires USCG approved fuel lines - so no off the shelf automotive or lawnmower lines should be in your craft , but you can always go for a higher standard and in case of tight and hot engine compartments it would be smart to get the best lines. If your fuel line burst under high pressure you would only blow up once - just is it worth saving a few pennies?
Aeroquip Stainless Braided Hose AQP hose is light and flexible with a stainless steel cover to resist corrosion and abrasion. AQP elastomer inner liner is rated to 1000 psi and operates from - 45 deg. to +300 deg. F. Compatible with both petroleum and synthetic lubricants, hydrocarbon fuels and liquid engine coolants. 







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