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Author Topic:   Modern Outboard Fuel Paths
jimh posted 09-12-2002 12:00 AM ET (US)   Profile for jimh   Send Email to jimh  
I was quite intrigued with the complexity of the fuel circuit on Yamaha's new 2003 Z250 250-HP HPDI engine. Let me describe it for you.

Fuel enters the engine via a hose from the tank. Then it enters a water/fuel separator. This is necessary because of the fuel injection; injectors do not like water, nor do the cylinders.

With the water removed, the fuel is routed to a fuel pump. This pump draws the fuel from the tank and through the water separator.

The pump output feeds a vapor separator tank. This removes any vapors from the fuel, providing just liquid fuel to the next stage.

The water-free, vapor-free fuel is now pumped to the high-pressure fuel pump, where it is pressurized to an amazing 1,000-pounds-per-square-inch. Think about that pressure. It must be maintained by the pump, yet it must be contained in the hoses and connections downstream of the pump. The entire fuel system beyond this point is operating at 1,000-PSI!

From here, the fuel flows into two fuel rails, one for each bank of cylinders.
Pressure between the two fuel rails is equalized by a pressure equilizer line.

Connected to the two fuel rails are the six fuel injectors. Remember, the fuel is being delivered to the injector pre-pressurized at 1,000-PSI.

Also connected to the system is a fuel pressure sensor to provide feedback to the electronic control module.

The fuel injectors spray fuel into the cylinder for each power stroke cycle. Thus if the engine is running at 6,000 RPM, there are 3,000 power strokes per minute, or 500 injector operations per cylinder per minute. In one hour there are 500x60= 30,000 injector operations. After 1,000 hours of operation, each injector will have operated about 30-million times (at full throttle). Maybe that is high, so let's say each injector operates only half that much, say 15-million cycles of operation. Remember this is at 1,000-PSI. The injectors are operated electrically and are controlled by a high-speed computer controller.

Now out in my garage is a 27-year old two stroke outboard. The fuel system on that engine is a bit simpler. Fuel enters the engine via a hose from the tank. It goes to the simple diaphragm fuel pump, which feeds the carburetor float bowls. The venturi effect of air through the carburetor throat pulls fuel through the metering jets into the cylinder intakes. This system still works quite well after 27-years. I wonder how the Yamaha HPDI system will be working in 27 years from now?

kglinz posted 09-12-2002 07:43 AM ET (US)     Profile for kglinz  Send Email to kglinz     
As I remember from High School auto shop your number of injector operations is off a little. On a 2 stroke it fires every time the piston starts down . Each injector operates 1 time per rev. 6000 rpm = 100 times per second on each cyl.
jimh posted 09-12-2002 09:04 AM ET (US)     Profile for jimh  Send Email to jimh     
Oops--I was thinking this was a 4-stroke engine. You're right, the injectors will fire twice as often as I described. That is 60-million injector cycles per 1,000 hours of operation.

Incidentally, my 4.6L V-8 automobile engine uses a pressurized fuel rail and injector systm, but it operates at a bit low pressure. The electric fuel pump in the fuel tank pumps fuel to the engine compartment where a pressure regulator delivers it to the fuel rails. In this system the pressure is maintained at about 40-PSI.

Maintaining the pressure at a regulated and constant value is necessary because the metering of fuel into the cylinder by the injector is done based on how long the injector is operated. The longer the injector is opened the more fuel. The fuel rail must remain at a constant pressure or the injector timing will not deliver the proper amount of fuel into the cylinder.

jimh posted 09-12-2002 09:08 AM ET (US)     Profile for jimh  Send Email to jimh     
I think I am still way off on this injector cycle. Each cylinder fires each revolution of the crankcase, so at 6,000 RPM there must be 6,000 injector cycles per cylinder per minute. In an hour there are 360,000 injector cycles. In 1,000 hours of operation there will be 360-million injector cycles. That injector must be well made to stand up to that kind of pounding.
tbyrne posted 09-12-2002 09:58 AM ET (US)     Profile for tbyrne    
And you wonder why people buy four-strokes?

I recently purchased a new motor and went with a four-stroke Yamaha 80 because I was concerned with the reliability of a DFI/FICHT/HPDI motor. I know it is somewhat heavier than a traditional two-stroke, but I was satisfied that its reliability would be comparable. The price difference was not as dramatic as is often reported ($8,000 vs. $6,500-7,000). It came with a five year warranty and has run very well thus far.

It works for me, but I am still amazed at how often four-strokes are attacked by many two-stroke fans. I didn't buy a four-stroke expecting to eventually save money on fuel bills. I bought it because I wouldn't touch a DFI/FICHT/HPDI motor until they have had many dependable years under their belt.

Therefore, I just had to choose between a four-stroke and traditional two-stroke. I prefer the four stroke - easy starting, no fumes, no oil mixing, no "is my VRO working?", no plug fouling, better for the environment (in my layperson's opinion). It sure doesn't leave a sheen on the water the way previous two-strokes have.

To each their own, I guess.

lhg posted 09-12-2002 01:26 PM ET (US)     Profile for lhg    
JimH - I think you have just described why OMC went bankrupt with the Ficht design. Yamaha HPDI's and Merc Opti's have also had some problems. At least the Opti's only operate at 80 psi. Mercury makes a big deal out their low pressure system.
Peter posted 09-12-2002 02:01 PM ET (US)     Profile for Peter  Send Email to Peter     
In my opinion, the view that OMC went bankrupt because of FICHT technology is overly simplistic. There were a lot of reasons why OMC went bankrupt, not all of which were FICHT related. However, FICHT might have been the final nail in the coffin.
jimh posted 09-12-2002 08:00 PM ET (US)     Profile for jimh  Send Email to jimh     
If you think the fuel system is complex, wait until you hear about the ignition timing, etc. These modern engines are loaded with sensors:

--throttle position sensor
--crankcase position sensor
--air temperature sensor
--engine temperature sensor
--atmospheric pressure sensor
--fuel pressure sensor.

If a problem crops up with any of these the engine performance will likely be degraded.

lhg posted 09-12-2002 08:07 PM ET (US)     Profile for lhg    
Yea Jim, but those are the same things that make the EFI's, 2 or 4-stroke, run so well, and they all seem highly reliable so far.

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