Compression ratios

Tom-D

Tom-D

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Tom-D
At what ratio would you NOT run auto fuel? in an aircraft engine.
 
Anything over about 8:1 compression ratio (and even that might be a bit too high if you stay with the original ignition timing), anything with a turbocharger, and any low wing airplane with an engine driven fuel pump that has to "suck" the fuel upward from the tank into the pump suction.
 
GRG55 said:
Anything over about 8:1 compression ratio (and even that might be a bit too high if you stay with the original ignition timing), anything with a turbocharger, and any low wing airplane with an engine driven fuel pump that has to "suck" the fuel upward from the tank into the pump suction.
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This will be gravity feed to an electric boost pump feeding a fuel injection unit. at 9.4 to 1 compression pistons from light speed engineering.
 
It's much more complicated than compression ratio, it really has to do with peak pressures in the cylinder (which have a lot to do with airflow). An IO-540-C4B5 vs. an IO-540-K are great examples. 8.5 and 8.7:1 compression (respectively), however the -C4B5 will use MoGas just fine, and the -K will not. Difference? The parallel valve cylinders on the -C4B5 flow much less air than the angle valve cylinders on the -K.

Ignition timing comes into play as well and there are all sorts of things that can be changed to impact things. You need to look at the engine as a complete system, not just one part of it.
 
Tom-D said:
This will be gravity feed to an electric boost pump feeding a fuel injection unit. at 9.4 to 1 compression pistons from light speed engineering.
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Tom, could you post more details about the engine?
 
Also, define "auto fuel". Some consider it being the lowest octane unleaded, others think of premium.

I second the request for more information on the engine. Experimental w/electronic ignition or certificated with magneto technology from 19th century?
 
kgruber said:
Mazda is able to run 13.5/1 on 87 octane car gas.
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There is a lot of complexity, technology, and modern design (especially combustion chamber) that go into that engine that does not exist in our engines.
 
Checkout_my_Six said:
fire it up....and see what happens. It's an experimental....:D
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You say that like you believe Experimental engines are free. $800 for a case reworked, $1200 for a set of pistons (with pins) $1500 for the crank, rods, and lifter reworked. 5k for the injection kit. Yeah sure, we would really like to see it blow up on the test stand.
 
Ted DuPuis said:
There is a lot of complexity, technology, and modern design (especially combustion chamber) that go into that engine that does not exist in our engines.
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Yup. Knock sensors that tell the computer it's time to retard the timing and enrich the mixture for a bit. O2 sensors. Mass airflow sensors. Air temp sensors. Lots of stuff never imagined when I learned to fly.
 
1200 for a set of cylinders? I'll have what he's having!
 
Ted DuPuis said:
It's much more complicated than compression ratio, it really has to do with peak pressures in the cylinder.
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let's use the proper terms.. BMEP is what you are stating as peak pressures.
 
Tom-D said:
let's use the proper terms.. BMEP is what you are stating as peak pressures.
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Brake Mean Effective Pressure is a good indicator of engine out power per unit displacement, but not an indication of the temperatures / pressures in the end gas region.
 
Tom, at 9.5:1 I would expect 91 octane auto fuel to burn without detonation (assuming the timing is correct and you are not running lean, i.e. < 13:1 and assuming it is actually 91 octane - we could have a long discussion on that )
I am not seeing detonation in my Chevy BB engines with less than 10:1 ratios and 91 octane fuel - however I do set the total spark advance carefully and check it on a regular basis. And I am more comfortable at a 'measured' 9.5:1 on a street engine. And I use only a branded fuel where I know the manager of the local fuel depot and he assures me he is not buying unbranded/generic tanker loads from the spot market and delivering them to branded stations.
The cylinder head and valve temperatures on a water cooled engine are far more predictable than for air cooled - so that is a factor that has to be considered in addition.

However, I would also wonder if you will get vaporization problems in hot weather - which is prime flying season. In a quick scan of their web site, other than a brief mention of subtracting return fuel flow electronically when calculating fuel burn (as opposed to measuring it), I see no details of the amount of return flow and how close to the injectors that bypass occurs. With auto gas having higher vapor pressures I would want a healthy rate of pump-through on the system right up to the spider as a means of keeping the fuel delivered at the injector as cool as possible. And have a heat shield between the injector lines and the engine.

Having the pump sucking up to the fuel pump is a negative - as mentioned above. Get the pump as low on the fuel tanks as possible to maintain some 'head'.
Other issues are: will you have a magneto back up / what about prop resonance/rpm testing / will you have a backup electrical system
And you are caught between wanting to run rich to avoid detonation and wanting to run on the backside of egt to keep the heads cool.
Darn, this experimental flying machine stuff is complicated.

cheers
 
Misread that one.

Good luck with your plane, you still plan on selling it once done?
 
carburated engine?.....with high compression pistons? I wouldn't even consider running mogas. Unless it was instrumented and tested with an variable timed electronic ignition.
 
Dan Thomas said:
Yup. Knock sensors that tell the computer it's time to retard the timing and enrich the mixture for a bit. O2 sensors. Mass airflow sensors. Air temp sensors. Lots of stuff never imagined when I learned to fly.
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Exactly.

+ multiport injection, shaped piston heads, VVT, and no cable with a red knob leaving it to the "pilot's" discretion to run it LOP in cruise.

Comparing a state-of-the-art automotive engine with the air-cooled Lycomings and Continentals most of us run in our aircraft, and expecting we can replicate the compression ratio/RON performance is specious
 
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Checkout_my_Six said:
carburated engine?.....
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Didn't you go to the SDS web site? That's a direct fuel injection. I haven't got my mind totally around just how it works. but simply put, it's constant pressure to the injector, and the computer tells each injector when to open, how long to stay open, and the computer will dictate timing as required by MAP & RPM
 
but....you called it an "0"-200....no? Where does the EFI send the fuel?...does an 0-200 cylinder have a port for direct injection?
 
Tom-D said:
Didn't you go to the SDS web site? That's a direct fuel injection. I haven't got my mind totally around just how it works. but simply put, it's constant pressure to the injector, and the computer tells each injector when to open, how long to stay open, and the computer will dictate timing as required by MAP & RPM
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Looks like a really nice injection set up Tom, especially the individual cylinder "tuneable" mixtures.
From what I can see looks like the carb is replaced with a throttle body, but where are the fuel injection points? Into the individual intake runners? Or is there a modified head?
 
Dr. O said:
Tom, at 9.5:1 I would expect 91 octane auto fuel to burn without detonation (assuming the timing is correct and you are not running lean, i.e. < 13:1 and assuming it is actually 91 octane - we could have a long discussion on that )
I am not seeing detonation in my Chevy BB engines with less than 10:1 ratios and 91 octane fuel - however I do set the total spark advance carefully and check it on a regular basis. And I am more comfortable at a 'measured' 9.5:1 on a street engine. And I use only a branded fuel where I know the manager of the local fuel depot and he assures me he is not buying unbranded/generic tanker loads from the spot market and delivering them to branded stations.
The cylinder head and valve temperatures on a water cooled engine are far more predictable than for air cooled - so that is a factor that has to be considered in addition.
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Aircraft engines have larger cylinders, slower piston speeds, higher cylinder and head temperatures. Detonation is something that takes time to happen, so anything that slows the combustion is pro-detonation. It takes time for the flame front to cross a large cylinder. A lower RPM allows more time. Lean mixtures take longer to burn. Hotter cylinders and heads raise the temperature of the charge. The fuel molecules, given enough heat, pressure (from the flame front's pressure rise), and time, begin to break down from stable, long-chain molecules to shorter, easily-ignited molecules, and they will self-ignite all at once if conditions get to that point creating a huge pressure spike. Detonation. It doesn't even take high compression to do it.
 
GRG55 said:
Looks like a really nice injection set up Tom, especially the individual cylinder "tuneable" mixtures.
From what I can see looks like the carb is replaced with a throttle body, but where are the fuel injection points? Into the individual intake runners? Or is there a modified head?
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The injectors / pumps / adapters look like they are for port injection. Direct injection takes significantly upgraded hardware - typically an engine driven high pressure pump in addition to the electric pumps.
http://sdsefi.com/contio200kit2.jpg
 
Checkout_my_Six said:
but....you called it an "0"-200....no? Where does the EFI send the fuel?...does an 0-200 cylinder have a port for direct injection?
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At present it is a 0-200 when the machine work is complete it will a EXP .... (I) O-200-
 
GRG55 said:
Looks like a really nice injection set up Tom, especially the individual cylinder "tuneable" mixtures.
From what I can see looks like the carb is replaced with a throttle body, but where are the fuel injection points? Into the individual intake runners? Or is there a modified head?
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The injectors will be installed in the intake elbows, as a modification or they install them into the intake tubes.
 
Capt. Geoffrey Thorpe said:
The injectors / pumps / adapters look like they are for port injection. Direct injection takes significantly upgraded hardware - typically an engine driven high pressure pump in addition to the electric pumps.
http://sdsefi.com/contio200kit2.jpg
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Your correct, this is a multi-port fuel injection, the fuel in this system does not get injected into the combustion chamber, (like a diesel) but the term "Direct fuel injection" comes from the old Stromberg Carlson pressure injected carbs.
 
So, you've got a modefied O-200 with FADEC and port injection. You have just about everything except VVT. I would think Premium car gas would run ok. Even at 9.4:1. Are you running mags, or distributor, or one of each? VIT would be a handy thing to have in this case. Don't know how that would work with a magneto.
 
I would call the manufacture of the EFI system and ask them how their system is intended to be used.....vs. asking a bunch of old bald fat guys on the internet.
 
bluerooster said:
So, you've got a modefied O-200 with FADEC and port injection. You have just about everything except VVT. I would think Premium car gas would run ok. Even at 9.4:1. Are you running mags, or distributor, or one of each? VIT would be a handy thing to have in this case. Don't know how that would work with a magneto.
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When you use the SDS system you will be using their dual electronic ignition. = no mag
 
Checkout_my_Six said:
I would call the manufacture of the EFI system and ask them how their system is intended to be used.....vs. asking a bunch of old bald fat guys on the internet.
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I've talked to them at great length. thread creep has gotten us to where you'd believe this question was about the SDS system.
when really it was about how much compression is too much to run low OCT fuel.
 
Tom-D said:
I've talked to them at great length. thread creep has gotten us to where you'd believe this question was about the SDS system. when really it was about how much compression is too much to run low OCT fuel.
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....as you've discovered....it depends. :D
 
Tom-D said:
I've talked to them at great length. thread creep has gotten us to where you'd believe this question was about the SDS system.
when really it was about how much compression is too much to run low OCT fuel.
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About 9:1 give or take. Depending on the rest of the systems. Could go as high as 12:1. So there ya go.
My Mercedes got more HP out of the 305 cubic inch engine in my S500 than Chevy did in their Corvette with the 427 cubic inch.
But it was done with FI, VVT, and VIT. And all done on pump gas.
 
Any of you using a modern engine to make an estimate for our engines, please stop. It is an invalid comparison.
 
Now, my aircraft engine with a 9.1:1 ratio runs just fine on regular auto gas.

But, the cylinders are a lot smaller than an O-200.
 
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