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:

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.

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.

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.

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

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.

Hello all, There are some new CFG params currently targeted for SU13 to help
developers resolve these issues. Please see: -Matt

Matt, these new parameters look awesome, with much improved customization! I’m
looking forward to testing them out!