The Fuel curve of a Personal Water Craft/JetSki or other engine can be represented in many ways. Perhaps the easiest way is to consider fuel usage of the engine over time. This representation will go along nicely with the flow rating of such things as Carburetors or Fuel pumps and even Fuel Injectors which have a rating in some flow unit/minute or per hour.
When speaking of flow ratings, we are usually speaking in terms of volume. For example fuel pumps are often rated in gallons or litres per minute at a specific pressure drop (head pressure) so we may find a fuel pump whose rating is in Gallons per hour at 45 PSI pressure. In terms of flow rate of an engine, the volume used over a period of time is a function of the Swept Volume displaced by the engine in a cycle and its number of cycles completed in the measured time period. In the case of a four stroke cycle engine, the number of cycles completed per minute is half the number of revolutions completed per minute as it takes 2 revolutions to complete a cycle.
As it pertains to Swept Volume of the Personal Water Craft (PWC) engine, if we assume that the engine is perfect we would expect to see the Cycles Per Minute times the Swept Volume (in some units) = Volume flow rate per minute.
As an example if we take a displacement of the Yamaha PWC at 1800cc and we have 10000 RPM then we have 5000 cycles per minute with each cycle using 1800 cc we then have 9,000,000 cc's per minute.
Lets look again at 2000 RPM same engine will be 1000 cycles at 1800 cc's per cycle or 1,800,000 cc's per minute.
You may note that 1,800,000 cc's is 20 percent or 1/5th of 9,000,000 and that 2000 rpm is also 1/5th of 10000 rpm. If engines were perfect at pumping air at all RPM's this relationship would perfectly describe the airflow and therefore fuel flow curve needed to properly fuel an engine like this.
Unfortunately, the engine is not a perfect pump. It is in fact a series of compromises depending on its intended usage. The Swept Volume (Geometric displacement) is not what the engine is able to acheive over its entire operating range. Tyically, we find that engines optimized for high speed operation have nearly or greater than their Swept Volume at high speed due to interia effects and suffer at low speed to sometimes as low as half of their Swept Volume. We are referring to Volumetric Efficiency. That is what percentage of the Engine's Geometric Swept Volume, is actually displaced on a per cycle basis.
Notice that we have now added "per cycle" to the equation. Now in order to solve for this new dimension "Volumetric Efficiency" we will have to assign a number based on the operating speed of the engine which represents it's percentage of Swept Volume that it actually displaces.
An example might look now like this;
1800(SV) X .80 (VE) X 5000 (Cycles Per Minute)
1440 X 5000
= 7,200,000 cc's per minute. This is quite different from what we have above and if plotted on a graph would certainly show us that the engine's airflow and therefore fuel flow requirements are not linear with increased speed.
Now a graphical representation of the Volumetric Efficiency of the engine, on a per cycle basis might look like this;
The peak representing the point in the RPM curve where the engine is most efficient (VE highest) with a slope downwards either side of that. At low speed (left side the VE might be quite low because of large overlap cams etc.
MoTeC systems contain a Fuel Map which is derived from this Volumetric Efficiency curve of the engine. This is because MoTeC systems must deliver fuel in the proper proportion to the air which is trapped in the cylinder at varying RPM's. More fuel will be needed to maintain a constant air fuel ratio at the VE peak than will be required to do so at any other point on the curve. If your system has been calibrated correctly, your main fuel table will be a direct representation of your engine's Volumetric Efficiency curve. It will vary in another dimension based on the Density of the air which it is breathing in past its throttle plate. This will give the fuel map a second axis usually defined as efficiency which is demand based. As we open the throttle the demand is increased and vice versa.
Your engine's fuel requirement will be in proportion to the air which it draws in. It should look something like this;
Remember that the fuel table is a direct representation of the air flow of the engine. It will be relatively smooth from point to point. It will only have peaks where the engine airflow peaks.
Some of you may now be realizing that there is a correlation between the engine torque and the Volumetric Efficiency. Yes, all other things being equal, the engine stands its best chance of creating peak torque at its highest VE.
Keeping all of the above information in mind, have a look at this Fuel Table below;
When speaking of flow ratings, we are usually speaking in terms of volume. For example fuel pumps are often rated in gallons or litres per minute at a specific pressure drop (head pressure) so we may find a fuel pump whose rating is in Gallons per hour at 45 PSI pressure. In terms of flow rate of an engine, the volume used over a period of time is a function of the Swept Volume displaced by the engine in a cycle and its number of cycles completed in the measured time period. In the case of a four stroke cycle engine, the number of cycles completed per minute is half the number of revolutions completed per minute as it takes 2 revolutions to complete a cycle.
As it pertains to Swept Volume of the Personal Water Craft (PWC) engine, if we assume that the engine is perfect we would expect to see the Cycles Per Minute times the Swept Volume (in some units) = Volume flow rate per minute.
As an example if we take a displacement of the Yamaha PWC at 1800cc and we have 10000 RPM then we have 5000 cycles per minute with each cycle using 1800 cc we then have 9,000,000 cc's per minute.
Lets look again at 2000 RPM same engine will be 1000 cycles at 1800 cc's per cycle or 1,800,000 cc's per minute.
You may note that 1,800,000 cc's is 20 percent or 1/5th of 9,000,000 and that 2000 rpm is also 1/5th of 10000 rpm. If engines were perfect at pumping air at all RPM's this relationship would perfectly describe the airflow and therefore fuel flow curve needed to properly fuel an engine like this.
Unfortunately, the engine is not a perfect pump. It is in fact a series of compromises depending on its intended usage. The Swept Volume (Geometric displacement) is not what the engine is able to acheive over its entire operating range. Tyically, we find that engines optimized for high speed operation have nearly or greater than their Swept Volume at high speed due to interia effects and suffer at low speed to sometimes as low as half of their Swept Volume. We are referring to Volumetric Efficiency. That is what percentage of the Engine's Geometric Swept Volume, is actually displaced on a per cycle basis.
Notice that we have now added "per cycle" to the equation. Now in order to solve for this new dimension "Volumetric Efficiency" we will have to assign a number based on the operating speed of the engine which represents it's percentage of Swept Volume that it actually displaces.
An example might look now like this;
1800(SV) X .80 (VE) X 5000 (Cycles Per Minute)
1440 X 5000
= 7,200,000 cc's per minute. This is quite different from what we have above and if plotted on a graph would certainly show us that the engine's airflow and therefore fuel flow requirements are not linear with increased speed.
Now a graphical representation of the Volumetric Efficiency of the engine, on a per cycle basis might look like this;
MoTeC systems contain a Fuel Map which is derived from this Volumetric Efficiency curve of the engine. This is because MoTeC systems must deliver fuel in the proper proportion to the air which is trapped in the cylinder at varying RPM's. More fuel will be needed to maintain a constant air fuel ratio at the VE peak than will be required to do so at any other point on the curve. If your system has been calibrated correctly, your main fuel table will be a direct representation of your engine's Volumetric Efficiency curve. It will vary in another dimension based on the Density of the air which it is breathing in past its throttle plate. This will give the fuel map a second axis usually defined as efficiency which is demand based. As we open the throttle the demand is increased and vice versa.
Your engine's fuel requirement will be in proportion to the air which it draws in. It should look something like this;
Some of you may now be realizing that there is a correlation between the engine torque and the Volumetric Efficiency. Yes, all other things being equal, the engine stands its best chance of creating peak torque at its highest VE.
Keeping all of the above information in mind, have a look at this Fuel Table below;
Actual Map extracted from client's Personal Water Craft from a well known PWC distributor
We can surmise a few things from a table like this.
- This engine has a very strange and illogical VE and therefore Torque Curve
- During calibration, something was affecting the flow of fuel into the engine resulting in a non linear fuel delivery requirement to match the engine's normal VE
- the calibrator did not do his job properly.
For Option 1 to be true, in the data logging, we would see that despite this silly looking fuel VE curve, the engine actually runs cleanly and manages to acheive the desired air fuel ratio under all conditions encountered of both RPM and Demand. If this is the case (in 10 years I have never seen this to be the case) then this fuel curve is spot on for the engine intended.
For Option 2 to be true, some value, most likely fuel pressure is going to be extremely erratic, battery voltage erratic or both. Other option would be a faulty lambda sensor used during calibration. Data logging should show this and an experienced and properly trained tuner should be able to spot this before he finishes the calibration.
For Option 3 - well we have to look no further than the Map itself. Anyone claiming to be a tuner as a profession, who turns out work of this nature is quite simply committing Fraud. Unless the calibration was free (in this case - tuner knows what his work is worth at least) you should fire your tuner and take your vehicle straight to a professional. This type of shoddy craftsmanship should not be tolerated by anyone in our industry.
The end result is you get what you pay for. MoTeC tuners like Shane Tecklenburg charge a fair price for the level of skill required to get the job done correctly. With 10 years of experience tuning MoTeC engine magagement systems in every conceivable venue, he has the skill, knowledge and patience to accomodate even the most challenging applications.
for more information email shaneteck@yahoo.com or call 714-318-5845 for a consultation on your system.
