In this guide will be presented the equation that I found to link the stoichiometric coefficient (the one that you see when looking the cylinders of your modular engine) to the ratio between the fuel manifold throttle and the air manifold throttle.

Please, if you find this guide useful, consider to leave a feedback or a positive review: the solution that I found is simple but it took my spare time during the last 3 weeks to obtain this equation, thank you.

During the last 3 weeks I read a lot of guides/discussions on Steam and Reddit about modular engines AFR (Air-Fuel Ratio) and I found what seems to be a well-known equation relating the stoichiometric coefficient and the corresponding AFR, for a given engine temperature:

AFR(s,T) = (14-2s)*(1-0.01T)+(15-5s)*(0.01T)

where:
AFR(s,T) is the Air-Fuel Ratio in function of the desired stoichiometric coefficient and engine temperature
s is the desired stoichiometric coefficient (a number between -1 and +1, use +0.2 to maximize engine efficiency)
T is the engine temperature measured in °C and clamped between 0°C and 100°C

If you do the math you will find that the equation above can be written in this shorter form:

AFR(s,T) = 14+0.01T-2s-0.03Ts

The only thing we miss, at this point, is the relationship between air pressure and the ratio between fuel throttle and air throttle: after some hypothesis and a lot of data collections, tests and curve fittings I found the following (and last) equation:

(F/A) = (6.88+(0.0625p))/AFR(s,T)

where:
F is the fuel manifold throttle
A is the air manifold throttle
p is air pressure at the air manifold inlet, capped at 90 (I prefer to clamp it between -1 and 90)

Don't forget to multiply the ratio obtained from the last equation by the air throttle value to calculate the fuel throttle you need!

F = A*(6.88+(0.0625*p))/AFR

Build your engine: doesn't matter the size (1x1, 3x3, 5x5) and the number of cylinders provided that you are using enough exhaust manifolds.
Complete it with battery, fuel tanks, throttle lever, starter key etc.

At this point we need:

• a temperature measurement system (a temperature sensor on the engine crankshaft or the 3rd channel of the composite output of one of your cylinders)
  
• a numeric value for the stoichiometric coefficient that you want to obtain (example: a keypad)
  
• a pressure sensor (can be avoided in case of a naturally aspirated engine) linked to the pipe between your compressor (if you have a supercharged engine) and the air manifold inlet

I haven't forgotten rps, air and fuel readings from the cylinder: we don't need them!

Now, in a few steps, we can build a simple microcontroller :

1. design a 3x2 microcontroller
2. add:
a numeric input for engine temperature to be linked to your temperature measurement system
a numeric input for air manifold pressure to be linked to the pressure sensor (if you have one)
a numeric input for your engine throttle
a numeric input for your desired stoichiometric coefficient (0.2 for maximum efficiency)
a numeric output for your air manifold throttle
a numeric output for your fuel manifold throttle
3. use a clamp between 0 and 100 for the temperature input, connect it to a first numerical function block f(x,y,z) at the x input
4. connect the stoichiometric coefficient input to the y input of your function block
5. put into the function block the following formula 14+(0.01*x)-(2*y)-(0.03*x*y)
6. connect the first function block output to the x input of a second function block
7.connect the air manifold pressure input to the y input of your second function block, by using a clamp between -1 and 90
8.put into the second function block the formula (6.88+(0.0625*y))/x
9.using a multiplier block, multiply your engine throttle input by the output of the second function block
10.link the output of the multiplier block to the fuel manifold throttle output
11.also link the the engine throttle input directly to the air manifold output

Turn on the key and watch your cylnders stoichiometric coefficient keeping the value you wanted.


Final hints:
-use the same number of air manifolds and fuel manifolds
-in case of a supercharged engine, if you experience some air manifold pressure instability: consider to use a variable valve, positioned between the "compressor" (pump) and the pressure sensor, that remains partially open and discharge the excess air into the ambient.

Here you can find a link to a simple microcontroller that works with my equations:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2786934052

Here there is a link to a test dyno to show how you have to connect the microcontroller above:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2786928037

A picture from a my microcontroller very similar to the one described above:

A special thanks to Lupus the Canine and his awesome "CSV logger" microcontroller
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2408195283

Another special thanks to all the people that wrote something on modular engines AFR and stoichiometry

• April 5, 2022

Manifold pressure reading must be capped at 90 in the equation (A/F)=((6.88+(0.0625*p))/AFR(s,T)