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WxMarc avatar image
WxMarc suggested Synoxys787 commented

Inaccurate Mixture Logic for Turbocharged Piston Engines

The engines.cfg file contains options to define a turbocharged piston engine as follows:

engine_type = 0 ; 0 = piston
turbocharged = 1 ; Is it turbocharged? 1 = TRUE

When creating an aircraft with these options, the fuel mixture behavior incorrectly varies with altitude. A turbocharged engine is capable of maintaining full power manifold pressure up to the turbocharger's critical altitude. Because of this, the fuel/air ratio will remain relatively constant with altitude (below the critical altitude) as long as the mixture lever is not adjusted.

Currently in MSFS, with engine_type = 0 and turbocharged = 1, the fuel/air ratio increases with altitude regardless of whether the airplane is above or below critical altitude. This results in loss of power and eventual inability to climb without leaning the mixture. I collected test data from the sim using the stock G36 Bonanza with turbocharged = 1 and critical_altitude = 18000. The data shows that fuel/air ratio varies with ambient air density:


This behavior is correct for a naturally-aspirated engine, but a turbocharger increases the manifold pressure (and therefore density) above the ambient value.

To make matters more unrealistic, fuel flow gradually decreases at higher altitudes when mixture is full rich. Here are some test data I collected by modifying the stock G36 Bonanza with turbocharged=1:

Fuel Flow (gph) and Altitude (Full Rich Mixture)

2,000 ft: 23.5 gph

3,000 ft: 23.6 gph

4,000 ft: 23.7 gph

5,000 ft: 23.8 gph

6,000 ft: 23.7 gph

7,000 ft: 23.1 gph

8,000 ft: 21.3 gph

9,000 ft: 19.4 gph

10,000 ft: 14.8 gph

11,000 ft: 7.7 gph

12,000 ft: 0.5 gph

I performed another test flight and manually leaned the mixture at 1,000 ft intervals until I found the maximum fuel flow. Here are the results:

Altitude: Max Fuel Flow:

2,000 ft: 23.6 gph at 94% mixture

3,000 ft: 23.7 gph at 91% mixture

4,000 ft: 23.9 gph at 64% mixture

5,000 ft: 24.0 gph at 66% mixture

6,000 ft: 24.1 gph at 54% mixture

7,000 ft: 24.2 gph at 52% mixture

8,000 ft: 24.3 gph at 50% mixture

9,000 ft: 24.4 gph at 48% mixture

10,000 ft: 24.5 gph at 44% mixture

11,000 ft: 24.6 gph at 42% mixture

12,000 ft: 24.6 gph at 40% mixture

To summarize: when turbocharged = 1 and mixture is full-rich, fuel/air mixing ratio increases with altitude and fuel flow *decreases* with altitude in MSFS. In reality, fuel/air mixing ratio and fuel flow should be relatively constant below the critical altitude.

The maximum fuel flow numbers reported above demonstrate that the MSFS turbocharged engine model is capable of producing full-power fuel flows at high altitude, but mixture must be leaned to realize that performance -- an unrealistic result.

As a simplistic approximation, it would be better to assume a constant relationship between mixture lever percentage and fuel/air ratio when turbocharged = 1 (in other words, fuel/air ratio would be independent of altitude).

For more advanced development, it would be useful to include options for behavior above critical altitude (Option 1: constant fuel/air ratio with altitude. Option 2: increasing fuel/air ratio with altitude above critical altitude). Fuel/air ratio will also depend on the density of air delivered to the cylinders. In a turbocharged engine, this is affected by compression heating and intercooler effectiveness.

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Flysimware avatar image
Flysimware commented

Hi Asobo team this is Momo from Flysimware and this will be very helpful for products like our Cessane 414AW Chancellor that is coming to the market place in a few weeks. For now we plan to use this 3rd party patch. Turbocharged aircraft need this as a type of air file. So that would be awesome to add has it already has been developed tested and works. Thanks.

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WxMarc avatar image
WxMarc commented

To clarify: the third party patch Momo is referring to is the WASM module which I created to produce more realistic mixture behavior in turbo piston aircraft. Here is a link to the github repository with the source code and compiled WASM modules:

If this is helpful in any way for improving the logic in the core sim, Asobo is welcome to use it.

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Chancellor avatar image
Chancellor commented

I fly a RAM 414A regularly. The fuel/Air mixture depends on fuel flow vs. upper-deck pressure measured as Manifold Pressure. The mixture control setting doesn't change with aircraft altitude but it does change with manifold pressure and temperature. Once climb power is set after takeoff in the RAM 414A, it rarely if ever changes as the aircraft climbs from the initial Climb Power setting made at about 1500 MSL all the way to my normal long-range cruise altitude of FL250.

The issue I have in the simulated 414A is I cannot lean the mixture at all. I'm new to MSFS 2020, so maybe there is an auto-lean setting somewhere.

My normal Mixture settings are:

Climb power = 2500 RPM, 35" MP, 27 GPH/engine at 1450 degrees EGT (Peak EGT + 150 degrees).

Cruise power = 2300 RPM 30" MP, 18 GPH/engine at 1475 degrees EGT (Peak EGT + 75 degrees)

These settings are constant, regardless of altitude, but vary slightly with outside air temperature. This is because the air density at any given manifold pressure will change slightly with temperature.

I hope this helps and doesn't add to confusion.

Best regards, Larry

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Synoxys787 avatar image
Synoxys787 commented

Just dropping by to show my support for this. Accurate turbocharged engine behavior would really make a huge difference for all presently available and future aircraft that use such an engine. I would love to see it implemented in the near future.

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