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Can someone explain the various types of procedures to start a fuel injected piston engine? What is a cold start, hot start, etc.?

I'm referring to a GA aircraft.

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  • $\begingroup$ Is there something other than hot start and cold start? Are you interested in procedures for starting? $\endgroup$ – Ron Beyer Feb 25 at 21:22
  • $\begingroup$ Sure, I would like to know a bit more in details about starting procedures in a fuel injected engine . $\endgroup$ – Mike no smith Feb 25 at 21:35
  • $\begingroup$ Fuel injected GA engines are pretty reliable, they don't need priming and aren't prone to flooding like a carbureted engine. Really, it's not much different than starting a car, just turn the key and it either starts or it doesn't. FYI, terminology such as "hot start" means something completely different for turbine engines. $\endgroup$ – Michael Hall Feb 25 at 21:49
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The key thing that makes FI engines different is how you prime them. You are using the pressurized fuel injection system itself to do the priming instead of pumping fuel into the intake ports using a syringe pump primer. You do this by turning the boost pump on to pressurize the system and setting the mixture to rich, which on a crude-ish mechanical FI system like the continuous flow systems used on airplanes, just starts spraying fuel in the injectors as long as the mixture is in, whether the engine is turning or not.

Two main differences from a carb:

  1. The priming procedure is one of timing using mixture, not counting pump strokes.
  2. You start the engine with the mixture in idle cutoff (because with the system pressurized and the mixture not at idle cutoff, it'll be spraying fuel in continuously and you'll soon have a little pond of fuel under the cowl). You don't advance the mixture until the engine catches.

As far as hot/cold starting goes, the basics are the same as a carbureted engine, more prime when cold, less prime when warm, no prime when hot. So for a cold engine that uses 6 strokes of a manual primer, with the FI engine it's pump on and mixture rich for some period like 5 seconds, then back to idle cutoff before you crank it. On a warm engine that uses 2 strokes, the FI version might be 1-2 seconds. You're doing the same thing, just with a different process.

Because it requires careful timing with prime and then keeping the mixture at idle cutoff until start, it's easier to screw it up and flood the engine by messing up the timing, or to be late with the mixture after it starts and it goes and dies on you. So they can be a little bit fussy until you get the hang of it. If you're bush flying on floats in something like an injected C-185, where you need to be able to know you can start right now once you push off from the dock, getting it right really becomes important and hilarity will often ensue (drifting toward rocks or moored boats in the breeze as you scream and curse) if you don't.

At the other extreme, because the injector distribution system is normally on top of the engine where all the heat collects, when an FI engine is hot on a hot day it can cause vapour lock in the fuel distributor and lines (even with the very low Reid Vapour Pressure of avgas) and they just don't want to start at all until the cowling cools down a bit. Not realizing this, you may decide that it needs priming, so you start priming when it doesn't need it and things have cooled down enough for fuel to start to flow again, and now it still won't start (the raw fuel smell with be a clue). They can be frustrating beasts.

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It all depends on the manufacturer. Please refer to your planes specific POH.

If you are confused about the terms here is a brief primer:

  • Normal Start - Anytime you start the engine under normal circumstances
  • Hot Start - Starting the engine under conditions when the engine is hotter than the average ambient air temperature. This could be caused by starting the engine shortly after shutting it down after a flight. The engine could remain hot for as much as an hour.
  • Cold Start - Depending on the manufacturer’s interpretation, it could mean the same as a normal start (in other words, not hot). It could mean starting the engine when the ambient air temperature and the engine (which has been sitting) are near or below freezing.
  • Flooded Start - When too much fuel has been introduced into the cylinders to accommodate stoichiometric ignition.
  • External Power Start - Starting without the use of an onboard battery operational.
  • Fuel Vapor Formation Start - May be similar to Hot Start in climates with higher ambient air temperatures. It purges the fuel lines of vapor. Vapor can cause vapor-lock (the inability to create sufficient vacuum in the fuel lines due to the compressibility/expandability of a gas (vapor) versus a liquid.

Here is part of what my POH for the Cessna 172S says:

STARTING ENGINE
In cooler weather, the engine compartment temperature drops off rapidly following engine shutdown and the injector nozzle lines remain nearly full of fuel.
In warmer weather, engine compartment temperatures may increase rapidly following engine shutdown, and fuel in the lines will vaporize and escape into the intake manifold. Hot weather starting procedures depend considerably on how soon the next engine start is attempted. Within the first 20 to 30 minutes after shutdown, the fuel manifold is adequately primed and the empty injector nozzle lines will fill before the engine dies. However, after approximately 30 minutes, the vaporized fuel in the manifold will have nearly dissipated and some slight priming could be required to refill the nozzle lines and keep the engine running after the initial start. Starting a hot engine is facilitated by advancing the mixture control promptly to 1/3 open when the engine starts, and then smoothly to full rich as power develops.
If the engine does not continue to run, set the FUEL PUMP switch to the ON position temporarily and adjust the throttle and/or mixture as necessary to keep the engine running. In the event of over priming or flooding, set the FUEL PUMP switch to OFF, open the throttle from 1/2 to full open, and continue cranking with the mixture in the IDLE CUTOFF position (pull full out). When the engine fires, smoothly advance the mixture control to full rich and retard the throttle to desired idle speed.
If the engine is under primed (most likely in cold weather with a cold engine), it will not start at all, and additional priming will be necessary. After starting, if the oil pressure gage does not begin to show pressure within 30 seconds in warmer temperatures and approximately one minute in very cold weather, stop the engine and find the cause before continued operation. Lack of oil pressure can cause serious engine damage.
NOTE
Additional details concerning cold weather starting and operation may be found under COLD WEATHER OPERATION paragraphs in this section.

This is what my checklist says for the Cessna 172S

NORMAL
THROTTLE......................................................................¼" OPEN
FUEL PUMP ..................................................................................ON

COLD ENGINE
MIXTURE ........................................................RICH 3-5 SECONDS UNTIL STABLE FUEL FLOW, THEN CUTOFF

WARM ENGINE
MIXTURE .................................RICH 1 SECOND THEN CUTOFF FUEL PUMP ................................................................................OFF BRAKES....................................................................................HOLD PROP AREA............................................................................ CLEAR MAGNETOS.............................................(ATP 8 seconds max) START MIXTURE .................................. (advance smoothly as engine starts) RICH

WHEN ENGINE STARTS
THROTTLE....................................................................... 1000 RPM OIL PRESSURE ................................(in 15 seconds) CHECK GREEN MIXTURE ......................................................LEAN FOR RPM RISE THROTTLE....................................................................... 1000 RPM AMPS (M BATT & S BATT).................................CHECK (positive charge) LOW VOLTS ANNUNCIATOR.........................CHECK (not visible) FLAPS..............................................................................................UP

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