1. Table of Contents

2. Introduction

This document contains information that may be useful to those operating the Prometheus software The purpose of the software is to aid in the setup of kart carburettors and kart chassis.

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3. Installation.

The software can be downloaded from the Prometheus website.

The software should be saved to a folder on its own. The software will install itself once run. A separate Installation Guide is available.

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4. Description of Software

The software consists of Explorer-style browser style interface with two toolbars. The LHS pane is called the navigation pane, and the RHS pane is called the detail view. Clicking on one of the nodes in the navigation pane causes the detail view to display the associated information.

The navigation pane is organised into engines and tracks. The track information is further organised into race and then session information. Note that:

• Each engine must have a unique name.

• Each track must have a unique name.

• Each race for the same track must have a unique date (E.g. "Summer Championship R1").

• Each session for a race must have a unique name (E.g. "Heat 1").

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6. Adding an Engine

It is good practice to treat the same engine equited with different needles as two engines.

For example, if a Senior-max engine is sometimes run with a K27 needle and sometimes run with a K98 needle, it is good practice to add 2 engines to Prometheus, say "GoodSeniorMax_K98" and "GoodSeniorMax_K27".

The reason for this is that the software may generate different pin recommendations for the same engine equiped with different needles. It is useful to compare the Prometheus recommendations with saved track data to see if the Prometheus constants should be adjuested.

You should always treat an engine/carb/exhaust combination as a single engine. It is good practice to choose names that reflect these combinations. For example, an engine name may look like "GoodSeniorMax_Carb02_8.5_Exhaust01_K98"

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7. The Weather Bar

The Weather command bar is used to enter Pressure, Temperature and Relative Humidity. The input values can be “incremented” using the up and down arrow keys. These inputs are used to calculate air density and dew point. The Relative Humidity is also used by the Grid views (see below).

7.1 Sea-level Atmospheric Pressure

The pressure input control has a button which displays a dialog which takes the current sea-level atmospheric pressure as reported by the Department of Meteorology and the track altitude. The calculated pressure at altitude is displayed and will be passed back to the parent pressure input control if not cancelled.

Note that the use of sea-level atmospheric pressure is not recommended. It should be used only as a last resort. It is generally better to walk the paddock to find someone with a barometer, even if it is not a very accurate one.

7.2 Icing Warning

Prometheus issues an icing warning when conditions indicate that icing inside the carburettor is a genuine possibility. Water may be present in the carburettor because it has condensed directly from the air or because of contaminated fuel. The water will pool at the bottom of the fuel bowl until it is suddenly drawn up into the main jet. If the water droplet is large enough, it will block the fuel flow and the engine will suddenly cut out. Annoyingly once the engine has died, the water droplet will fall back into the bowl and the engine may start and run again for a short time before failing.

It is much better to take precautions against water in the fuel when Prometheus indicates Icing is a possibility.

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8. The Engine Bar

The engine bar displays engine and model information on the main menu.

Engine characteristics are displayed in the top row of the bar

• The engine dropdown control contains the last engine selected in the navigation panel. This can only be changed by clicking on a different engine node in the navigation pane.

• The Needle control allows the user to select a needle type to use in the jetting calculations.

• The Plug brand control displays the brand of plug used in the engine.

• The Float/Idle Jet control displays the float/idle jet combination used in the engine.

• The Float Height control displays the carburettor float height.

The idle jets have a minimal effect on full-throttle operation, but do have a significant effect on mid-range preformance.

The float height also has a minimal effect on the mixture. The float height should be increased if the engine misses while cornering; if the float height is too low the fuel may aerate and not flow correctly through the jets.

Prometheus makes no distiction between 8.5 and 12.5 carburettors.

Model characteristics are displayed in the bottom row of the bar

• The Max RPM control displays the maximum RPM that is expected to be attained by the engine for the purposes of generating a recommendation. If the engine attains a higher than normal engine speed, it will need more fuel than normal and may need a larger jet. If the engine attains a slower than normal engine speed, it will need less fuel than normal and may need a smaller jet.

  The maximum RPM attained by an engine is affected by the track, and can only be determined by visiting the track. If you have no information about Maximum RPM, then set it equal to the Base RPM. The recommended jetting will not be too far out and Maximum RPM can be updated after the first run.

• The Base RPM control displays should be the maximum RPM obtained at most tracks. It does not have to be exact. The Base RPM is used in the calculation of some detail views.

• The constants a1, b1, b3 are shown in separate controls. These constants are unique to each engine/carburettor/exhaust/Base RPM combination. Engines of the same type should have similar values.

Default value are supplied by the application when an engine is created, however the user may adjust these constants based on experience using the engine.

The values in the input controls can be “incremented” by using the up and down arrow keys.

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9. Application Data

Application data is stored in the Ventimore subdirectory in the standard Windows Application Data directory. Data files should not be edited by hand. Prometheus uses a non-standard XML parser for portability reasons.

9.1 Sharing data with other applications

The data stored by Prometheus may be shared with other applications. It is only read when the application starts up, and only written when the application closes down.

It may be useful to change the application data directory. For example, the user may want to publish data to the cloud using Dropbox or a similar product.

The user can change the data directory by selecting "Options" and then "Custom Settings" from the main menu. The displayed dialog displays the data directory in the text box labelled "Data Directory". If the data directory is changed in the dialog, the existing data file will be copied from it's current location to the specified location. If this cannot be done, the dialog will refuse to close. If the problem cannot be resolved the user should exit from the dialog using the "Cancel" button.

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10. Model Limitations

The models presented above have a number of limitations. These include:

1. The models are extremely crude and only a rough approximation to the real world. More sophisticated models such as CFD are possible but involve considerable time and effort and it is unclear whether practical CFD models would produce results to the desired accuracy.
   
   

2. There are significant errors associated with the inputs to the model. A 1 degree Celsius change in temperature represents a change of 0.3%, a 1 mb change in pressure presents a change of 0.1%.

   Commerical thermometers can be assumed to be accurate to roughly 1 degree Celsius (0.3%), however a little experiment shows that moving a thermometer from location to location can change the recorded temperature significantly. It is probably better to assume a practical accuracy of 3 degree Celsius (1%), even within shaded locations.

   The air temperature above track directly exposed to the sun will differ from that shaded from the sun, further adding to the uncertainty over the correct temperature input.

   Atmospheric pressures appear to be more stable input, however a clogged carburettor filter will have same effect as a drop in atmospheric pressure, restricting the airflow and decreasing the density of the airflow into the carburettor.

   Note that you should never use any pressures based on information supplied by the Department of Meteorology or other government department. The supplied pressures are processed averages and not suitable for optimal carburettor tuning.

   The change from a 162 jet to a 165 represents approximately a 7% increase in flow.

   The result is that Prometheus should be able to produce a definite recommendation over a wide range of temperature, pressures and humidities, however the Prometheus recommendation may be less accurate near transition points. Prometheus contain views which show when 1% errors in the temperature and pressure may result in differing jetting recommendations.

   If you find that the current weather falls into one of these transitional areas you should be aware that the recommendation may not be optimal.

3. The models are hard to calibrate. It is necessary to establish the constants a1, b1, b2 before the models can be reliably used, however the constants vary from engine to engine.

   One possibility is to measure the components of the carburettor and estimate the values of the constants from measurements. The problem with this approach is the quantities involve are quite small and are very difficult to measure accurately. It is possible to make good estimates of the constants using a dynamometer, but most karters will not have access to one, or be willing or able to pay for access.

   An alternative is to use on-track data. Unfortunately even with large amounts of on-track data it may still not be possible to calibrate the model correctly since jet sizes and pin positions represent discrete data; an engine may operate best using a single jet over a reasonable large range of changing conditions before there is a transition with the engine operating better with a jet one size larger or smaller. The point where one jet becomes better than another is not always obvious.

   Small changes to the exhaust or carburettor may have a significant effect on the constants. (See recommendations in "Adding an Engine").

   It is for this reason that users can manually adjust the constants to reflect their own experience.

4. The mid-range carburation is affected by the driver and track. Rotax engines in particular do not respond well to fully opening the throttle at low or mid revs. If the driver does fully open the throttle, the vaccuum in the main throttle body drops dramatically while the jet remains fully open. The pin setting become irrelevant and the mixture will almost certainly not be optimal.
   

5. Pit lore says the pin position affects the flow at full throttle; increasing the pin position marginally enriches the mixture. The “Dellorto Area Rule” certainly suggests that the pin can produce back-pressure, however no matching logic has been implemented in the application.

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11. Practical Advice

Software such as Prometheus cannot guarantee that they will always produce the best carburettor settings, particularly if you do not have enough track data to calibrate the model.

It is sometimes possible to determine whether the mixture is correct by reading the spark plug. There are problems: (i) Different fuels burn differently so that moving from normal pump fuel to racing fuel can change the colour of the plug. (ii) The plug only reflects conditions over the last lap or so. If there is a long slow down lap and the kart trundles slowly through parc ferme before coming to rest, the plug will not reflect the conditions in the engine on a racing lap.

Do not attempt to read the plug if you do not have the correct plug installed in your engine. If the plug is too cold, the fuel may not burn properly around the plug and the plug will look too oily around the plug rim. If the plug is too hot, the engine may appear to be running lean.

You can establish the correct main jet by starting with a jet that is too rich and testing the performance on the track. Keep reducing the jet size until the engine starts to miss running down the straight. The correct jet is the smallest jet that does not cause a miss. Note however, the engine may misfire on the first few laps simply because it is cold, and then stop missing once it has warmed up. If this happens, the main jet is perfect and the engine will run very well.

If the jetting is wrong, it is often obvious to the trained eye. Other karts will simply be faster to accelerate out of the corners and/or faster down the straights. Sometimes the performance differential is dramatic.

Many teams just seem to know the jetting from long experience; joining a professional karting team is an effective, if expensive, way of providing the correct jetting.

Prometheus should not be seen as a replacement for inspecting the spark plug or other methods of checking the fuel mixture. Even if you prefer more manual methods, Prometheus can let you know how much changing conditions will affect your existing jetting.

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Appendix - Menus

Main Menu

The following main menu are displayed by the application.

Context Menus

The following context menus are displayed by clicking on various objects in the navigation pane.