Why do we startup and shutdown piston airplane engines the way we do?

MassPilot

MassPilot

Cleared for Takeoff
Joined
Jun 11, 2012
Messages
1,089
Location
Melbourne, FL
Display Name
Display name:
FloridaPilot
I'm referring specifically to the fuel injected C172SP's I fly. Why is it proper to shutdown the engine by pulling the mixture and starving it of fuel, rather than shutting off the ignition like every other vehicle I've ever operated?

Why do I need to prime the engine before I start it? Is it because I burned all the fuel left in the manifolds and combustion chambers when I last shut it down? I would think a car engine would not have any fuel vapor left in the system when it's been sitting for a while, but they don't require priming.

Why is it proper to start the engine with the mixture cutoff, then advance it to full rich as soon as it catches?

I've always followed these procedures without thinking too much about why, but now I'm curious and want to understand it better. Thank you.
 
You don't want to leave any fuel in the cylinders, just in case the magneto grounding fails and the mags stay live. We often move the prop slightly when we move airplanes. More than one person has been killed when the mags have been left on, or have failed.

You prime the engine before start because you cut the mixture off on shutdown, so there is no fuel in the cylinders to aid the start process.

Starting the engine with the mixture in cutoff is a technique for fuel injected engines. I don't know the exact answer to WHY, but I would guess it has something to do to prevent flooding while cranking the engine.
 
you do have to "prime" your car, or rather the computer does it for you. It doesn't start until it starts injecting fuel. If you are old enough to remember carbeurated cars, you should recall pumping the accelerator pedal before a cold start. That was to pump some fuel inth the intaka manifold with the accelerator pump. Just like in your cessna.
 
Car electric fuel pumps prime in cars when you turn your key to the acc position before it starts.
 
My Harley self-primes its injectors. Don't hit the start button till the check engine light goes out. Then it will fire right up and you can ride away right away, unlike the carb versions. No warm-up required but I do it anyway.

David
 
You mean the check engine light doesn't always stay on???? (LoL)
 
SAFETY

Cars don't have propellers
All fuel injected engines require priming, cars do it automatically
Carburetors on aircraft engines are up-draft, located underneath, so priming is still required.
 
Because airplane engine technology hasn't changed in over 40 years.
 
MassPilot said:
Why is it proper to start the engine with the mixture cutoff, then advance it to full rich as soon as it catches?
Click to expand...

Every airplane is different, and you'll have to find and use the technique that works for you - but I have a different technique for my injected C172.

On a cold start, it's mixture and throttle full forward, hit the boost pump until I see the first wiggle of the fuel flow gauge (indicating pressure at the injector divider) then pump off, throttle closed and crank - it will catch on the second blade everytime and idle nicely. If you overprime and flood the engine, then you will need to use the hot-start technique.

On a hot start, it's as you say - mixture cutoff and throttle open, crank and on the first cough from the engine shove the mixture in first then pull the throttle back, I usually catch it somewhere between 600 and 800 rpm but you have to move your hand fast.
 
Modern cars also have interlocks. On automatics you cannot start unless the transmission is in "Park", and on manuals you cannot start unless the clutch pedal is pressed fully.
 
SkyHog said:
Because airplane engine technology hasn't changed in over 40 years.
Click to expand...

This is the one that makes the most sense to me. After all, why do we even have to still be bothered with priming or the mixture? Seems like with modern technology, mixture would be automatically adjusted for the current conditions,
i.e. density altitude:dunno:
 
redtail said:
This is the one that makes the most sense to me. After all, why do we even have to still be bothered with priming or the mixture? Seems like with modern technology, mixture would be automatically adjusted for the current conditions,
i.e. density altitude:dunno:
Click to expand...

It's less necessary on an airplane because the engine pretty much runs at one operational point 90% of the time in a given flight, unlike a car engine that constantly changes conditions every minute if not second.

I only really f**k with my prop and mixture twice on a normal flight. First shortly after takeoff, then upon reaching cruise altitude.
 
redtail said:
Seems like with modern technology, mixture would be automatically adjusted for the current conditions, i.e. density altitude:dunno:
Click to expand...

Mechanical computers that did it for fuel injected engines are an old hat. For example, Fw-190 had that in 1940. But pilots of P-51s and Yak-9s have beaten them with manual mixtures, because the cost wins the war, not the sophistication.

The good old Rotax 912 has altitude-compensating carbs which do not even have a mechanical computer, just an aneroid chamber that moves a needle. Since there's no mixture control, you shut it down with the ignition key. Last year B-R introduced a computerized version which gets rids of choke. If you insist on this, you can buy an LSA.
 
airguy said:
Every airplane is different, and you'll have to find and use the technique that works for you - but I have a different technique for my injected C172.

On a cold start, it's mixture and throttle full forward, hit the boost pump until I see the first wiggle of the fuel flow gauge (indicating pressure at the injector divider) then pump off, throttle closed and crank - it will catch on the second blade everytime and idle nicely. If you overprime and flood the engine, then you will need to use the hot-start technique.

On a hot start, it's as you say - mixture cutoff and throttle open, crank and on the first cough from the engine shove the mixture in first then pull the throttle back, I usually catch it somewhere between 600 and 800 rpm but you have to move your hand fast.
Click to expand...

The procedure I was taught is to set the mixture to cutoff and the throttle 1/4" in.

For cold start only, I prime by turning on the boost pump, mixture full rich for three seconds, mixture cutoff, and boost pump off. No priming for hot start.

For both hot and cold start, my next step is to crank the starter until the engine catches and immediately shove the mixture to full rich, followed by backing the throttle out to 1000 rpm.
 
MassPilot said:
The procedure I was taught is to set the mixture to cutoff and the throttle 1/4" in.

For cold start only, I prime by turning on the boost pump, mixture full rich for three seconds, mixture cutoff, and boost pump off. No priming for hot start.

For both hot and cold start, my next step is to crank the starter until the engine catches and immediately shove the mixture to full rich, followed by backing the throttle out to 1000 rpm.
Click to expand...

The 172S POH says to do it airguy's way.

The 177RG POH says to do it your way.

Both make for fine cold starts on their respective airplanes.

Though I've found that cramming the mixture to full rich can wait a second or two. I generally tune the throttle first. Haven't had an engine die yet....

And I like to set the parking brake especially for a flooded start, just in case the engine races.
 
SkyHog said:
Because airplane engine technology hasn't changed in over 80 years.
Click to expand...

Fixed that for ya -

The 'reason' is that back when they invented magneto tech the grounding technology was not very good because the airplanes were made of wood- the only metal was in the engine itself - and some in the frame - perhaps.

So there was no reliable place to ground the mags - thus - they invented shutting off the fuel.

Certainly many of the early fighter aircraft used an on/off mag switch to reduce speed to land - but stopping the motor completely required killing the fuel. . .
 
comanchepilot said:
...The 'reason' is that back when they invented magneto tech the grounding technology was not very good because the airplanes were made of wood- the only metal was in the engine itself - and some in the frame - perhaps....
Click to expand...

You're not just making this up are you? Because a magneto only needs to be grounded to it's own case to disable it.
 
Silvaire said:
You're not just making this up are you? Because a magneto only needs to be grounded to it's own case to disable it.
Click to expand...

True, but the wiring for same runs to the cockpit switches and have been known to break, usually in the cowl due to engine vibration.

It's also a good idea before shutdown to switch the mags from both to left then right and back to both. That will tell you if you have a broken P lead and a hot mag.
 
JHW said:
you do have to "prime" your car, or rather the computer does it for you. It doesn't start until it starts injecting fuel. If you are old enough to remember carbeurated cars, you should recall pumping the accelerator pedal before a cold start. That was to pump some fuel inth the intaka manifold with the accelerator pump. Just like in your cessna.
Click to expand...

Stepping on the gas pedal also set the choke if the engine was cold. Yes, I'm old enough to have driven such dinosaurs. Heck, the starter switch was integral with the gas pedal in my 1954 Buick Special. Once the engine was running manifold vacuum locked the starter switch out. Turn on the ignition switch, step on the gas and the engine started. Watched a mechanic in the early 1970s darned near break off the key trying to start that old tank. Finally took pity and told him the starting sequence. :D
 
MAKG1 said:
The 172S POH says to do it airguy's way.

The 177RG POH says to do it your way.

Both make for fine cold starts on their respective airplanes.

Though I've found that cramming the mixture to full rich can wait a second or two. I generally tune the throttle first. Haven't had an engine die yet....

And I like to set the parking brake especially for a flooded start, just in case the engine races.
Click to expand...

I found a copy of the 172S POH that has an Engine Start checklist that matches what I do.
 

Attachments

  • checklist.png
    checklist.png
    58.6 KB · Views: 26
Norman said:
It's also a good idea before shutdown to switch the mags from both to left then right and back to both. That will tell you if you have a broken P lead and a hot mag.
Click to expand...

Just turn the mag switch to Off for a second to see if the engine starts to die, then back on and pull the mixture. But only go to Off if at idle; doing it at higher RPM can cause a big bang in the muffler and maybe crack something.

On the subject of carbs: Cars with carbs had chokes to make the intake suck extra fuel for starting. Chokes in some airplanes many years ago were a place for ice to accumulate, so most modern aircraft don't have them. Cars avoided ice by having a constantly-modulated carburetor heat system, but that cost a bit of power and mileage in most conditions.

There are engines being built now that have electronic fuel and ignition controls that will do it all just like modern cars. Lycoming's iE2 is one.

http://www.google.ca/url?sa=t&rct=j...fiGZ9FJ7GAaHh57mw&sig2=29WdrL-sKiBbHDwIWZtHsg

It's a pdf. Wait for it to load.

Dan
 
Ghery said:
Stepping on the gas pedal also set the choke if the engine was cold. Yes, I'm old enough to have driven such dinosaurs....:D
Click to expand...

That's what we called an "automatic" choke. I'm old enough to remember having to pull a knob on the dashboard and then push it back in when you got it warmed up a bit.
 
Norman said:
True, but the wiring for same runs to the cockpit switches and have been known to break, usually in the cowl due to engine vibration.
Click to expand...

Absolutely. When I bring an airplane in the hangar for an annual the first thing I do is disconnect all of the spark plug leads and they don't get hooked up again until it's ready to be rolled outside. At all other times I treat the prop as if the mags are hot and it's ready to start. It's not just broken p-leads, people forget to turn the switches off - myself included.
 
MAKG1 said:
The 172S POH says to do it airguy's way.

The 177RG POH says to do it your way.

Both make for fine cold starts on their respective airplanes.

Though I've found that cramming the mixture to full rich can wait a second or two. I generally tune the throttle first. Haven't had an engine die yet....

And I like to set the parking brake especially for a flooded start, just in case the engine races.
Click to expand...

I believe the 172S is fuel injected, the 177RG is carbureted.
 
I just Googled Cessna 177RG POH. Paydirt on first result. It specifies mixture idle cutoff for engine start, rich when engine fires. Just like every other fuel injected engine.
 
Not sure if someone else posted this, but another reason for cutting fuel off while the engine is running is to keep the fuel from washing the oil off the cylinder walls. ;)
 
dmspilot said:
I just Googled Cessna 177RG POH. Paydirt on first result. It specifies mixture idle cutoff for engine start, rich when engine fires. Just like every other fuel injected engine.
Click to expand...

That's correct, but all you need to do is just ease the mixture in once it begins to fire, don't rush going full rich or you risk flooding it.

For C177RG hot starts the Cardinal Flyers Online recommended procedure works every time:

When you're ready to shut down run the rpm to 1200, then pull the mixture to cut off.

Go about your business.

Get back in the airplane. DO NOT TOUCH THE THROTTLE OR MIXTURE. Run your normal preflight and prestart checklist.

When you are ready to start the engine DO NOT TOUCH THE THROTTLE OR MIXTURE.

Just turn on the master and rotate the key. The prop will begin to rotate.

After the 3 or 4th blade the engine will begin to fire. Every time. Now just ease the mixture forward, and be ready to reduce the throttle once it fires all cylinders.

Works every time.
 
MassPilot said:
I'm referring specifically to the fuel injected C172SP's I fly. Why is it proper to shutdown the engine by pulling the mixture and starving it of fuel, rather than shutting off the ignition like every other vehicle I've ever operated?
Click to expand...
The engine will happily operate without electrical power (by design). If the magneto is not properly grounded than it will still be live when you turn the prop over (and if there is fuel in the cyl it can easily start).

MassPilot said:
Why do I need to prime the engine before I start it? Is it because I burned all the fuel left in the manifolds and combustion chambers when I last shut it down? I would think a car engine would not have any fuel vapor left in the system when it's been sitting for a while, but they don't require priming.
Click to expand...
Your car has intelligent fuel metering, and the ECU performs this function automatically.

MassPilot said:
Why is it proper to start the engine with the mixture cutoff, then advance it to full rich as soon as it catches?
Click to expand...

I imagine you wouldn't do that unless you were flooded, typically you start it rich (at least in everything I've flown). I've only flown carbs though so maybe this is different.
 
rcpilot said:
I imagine you wouldn't do that unless you were flooded, typically you start it rich (at least in everything I've flown). I've only flown carbs though so maybe this is different.
Click to expand...

Injection is different.

In a carburetor generally, the air flow creates a vacuum in the venturi which pulls fuel into the manifold. At closed throttle there is an idle circuit that uses vacuum under the throttle plate to pull the fuel in since there is not enough air flow through the venturi to create the necessary injection.

With the Bentdics mechanical fuel injection, a venturi is used to create a signal during off idle operation, but at closed throttle, there is an idle circuit that will just **** fuel into the manifold even if the engine is not running (assuming that the mixture is not cut off). So if you pull the mixture to full rich during crank (or even with the engine stopped) you get way too much fuel in the manifold. Also, this is a reason for pulling the mixture in an injected engine to shut it down - leaving the mixture out and shutting off the ignition would just flood the engine for the next start.

http://www.precisionairmotive.com/Publications/15-812_b.pdf

TCM fuel injection differs in implementation detail, but not the overall process.
http://www.kellyaerospace.com/articles/ContinuousFlow.pdf


Now, in the automotive world, it is not enough that a car starts every time and does not stall after starting. Some (if not all) auto companies have standards for not just the crank time, but the variation in the crank time. If it takes 1/2 second to start one day and 1 1/2 seconds the next, you generate customer complaints and you get poor "quality" ratings in things like J.D. Powers so you have to set up the engine controls so that the variation in crank times is less than about 1/2 second - this means precise control of the fuel injected into each and every cylinder for each and every intake event as a function of engine temperatures, ambient temperatures, engine speeds, number of intake events, etc. etc. etc.

If an auto company put the Bentdics fuel injection system on their engines they would go out of buisness in about three days.
 
The flip side is that if you lose electrical power in your car, it quits. I've had three unrelated faults do that in my VW over the years Only one was in the ignition circuit (and it was the coil primary, which you don't even have in an airplane).

Airplane engines are designed to fail running, for obvious reasons. This makes fuel and ignition management completely different from a car, and it is not at all reasonable to expect them to operate the same.

Aside from poor Bendix injection behaviors at low throttle, another reason to crank at idle cutoff is that it makes it easier to find a combustible mixture. While cranking, you consume fuel (blowing it out the exhaust) and reduce the mixture. If you start rich, you'll find a combustible mixture that way.

If you've ever tried to start a carbureted engine with the mixture at idle cutoff, it takes the better part of 10 seconds for it to die. This procedure isn't really necessary for carburetors under normal conditions, but it is used for flooded starts.
 
I'll repeat it again - the reason is SAFETY

You have a 300-500 cubic inch engine fired by magnetos that have to be purposely shorted to stop working and connected to the crankshaft at the front is a 72 inch wide metal propeller that allows you to easily hand crank the engine through a combustion cycle.

Furthermore, it's an engine that requires you to yell "CLEAR PROP" when starting even under normal circumstances to avoid killing anyone.

So having a fuel charge in the manifold and cylinders of such an engine would be pretty idiotic don't ya think?
 
Absolutely. You need air, fuel, spark and compression to run. Compression is not easy to do away with. Neither is air; even a "sealed" cylinder has piston ring gaps. Spark is problematic because it is designed to fail hot. That leaves fuel. I'd much rather have to fuel starve the engine on shutdown than to have a broken ground wire cause the engine to quit in the air. The engine environment is quite hostile to wiring.
 
Geico266 said:
Not sure if someone else posted this, but another reason for cutting fuel off while the engine is running is to keep the fuel from washing the oil off the cylinder walls. ;)
Click to expand...

That is exactley what I was told too, makes sense to me anyway. In my Tcraft, shut down procedure is to bring RPM's up to about 1,000 then mags off, and wide open throttle. If I know I wont be flying for a week or two I cut the fuel and let it run dry that way.
 
MAKG1 said:
The flip side is that if you lose electrical power in your car, it quits. I've had three unrelated faults do that in my VW over the years Only one was in the ignition circuit (and it was the coil primary, which you don't even have in an airplane).
Click to expand...

It turns out that it isn't particularly difficult to fix that shortcoming...

http://www.flyrotax.com/enginesImpr...ressum/912-iS-100hpImpressum/Description.aspx

Airplane engines are designed to fail running, for obvious reasons. This makes fuel and ignition management completely different from a car, and it is not at all reasonable to expect them to operate the same..
Click to expand...

As someone who has worked on engine control systems for over thirty years, I think it's quite reasonable to expect reliable operation from an aircraft engine.
 
Capt. Geoffrey Thorpe said:
It turns out that it isn't particularly difficult to fix that shortcoming...

http://www.flyrotax.com/enginesImpr...ressum/912-iS-100hpImpressum/Description.aspx



As someone who has worked on engine control systems for over thirty years, I think it's quite reasonable to expect reliable operation from an aircraft engine.
Click to expand...

Reliable, yes. Identical, no. In what sense is cutting fuel for shutdown unreliable? Seems very effective to me.

You cut fuel in your car as well on shutdown. Injectors do not open when power is removed. There is no need for anti-Dieseling systems in electronic fuel injected cars, like there was on carbureted cars. A high idle speed setting and hot temperatures will not cause Dieseling on a fuel injected car.

A 1970 Nova used pure spark deprivation to stop the engine -- like many cars of the day, there was a mechanical fuel pump driven by the engine, so fuel would keep flowing while the engine shut down. If the idle speed was set too high, the shutdown would take a while. If temperatures were sufficiently high, it would require killing it with the clutch and parking brake, usually followed by an obnoxious backfire.
 
Last edited:
You must log in or register to reply here.
Back
Top