Under-cowl heat creates vapor lock
Carburetors are simple, reliable fuel metering devices that haven't changed appreciably in the past 70 years. But they have a couple of drawbacks, notably an inability to meter the fuel/air mixture as precisely as fuel-injection systems, and a tendency to ice up under certain atmospheric conditions. In spite of these proclivities, there are few pilots who haven't enjoyed many safe flights behind carbureted engines. In fairness, the inability of carburetors to meter equal amounts of fuel to all cylinders has more to do with the rudimentary induction systems installed on many aircraft engines than with the carburetor's shortcomings.
One thing you can be sure of, however, is that carbureted aircraft engines are a lot easier to start when hot than their fuel-injected counterparts. In most cases all that's required is to crack the throttle to the normal idle position, turn the key to start, and wait until the engine turns over five or six propeller blades. If the engine hasn't started after a few seconds, pump the throttle rapidly once to force fuel into the intake system via the accelerator pump that is built into all but the smallest engine carburetors.
Heat is the enemy
The main reason that fuel-injected engines are hard to start is that these systems deliver their fuel charge to the individual cylinders through extremely small stainless steel lines. These lines heat up during operation and, being steel, don't quickly cool after engine shutdown. The steel lines radiate out to the cylinders from a single distributor valve. These components are located between the top of the engine and the upper cowling skin—a spot where temperatures can easily exceed 200 degrees Fahrenheit after engine shutdown. These high temperatures result in the vaporization of the fuel in the lines, and often in the other components of the system, which causes vapor lock.
Vapor lock
Avgas is volatile; that is, it will evaporate easily. Every pilot who has ever taken a fuel sump sample and then disposed of the sample by flinging it into the air can attest to the fact that fuel does evaporate quickly. The three things that contribute to the formation of vapor lock are reduced fuel pressure, increased fuel temperature, and excessive fuel turbulence. Fuel-injection systems are designed to reduce turbulence-induced vapor, but there's little that can be done about reduced fuel pressures and increased heat.
Lowered fuel pressure and high engine-compartment temperatures will always cause vapor formation to some degree. If the ambient air temperature is high, the engine has heat-soaked for 15 to 20 minutes after shutdown, and the landing was made at a high-elevation airport, then the conditions are perfect for fuel-injection-system vapor problems.
Nothing in aviation is as frustrating as grinding the starter for minutes at a time without as much as a sputter, bark, or pop out of an engine that just successfully carried the pilot and his family for hours. More than one spouse has shied away from flying in the family's new high-powered, fuel-injected airplane because of a loss of confidence bred by the pilot's inability to get the engine started when it's hot. Not only is the inability to hot start a fuel-injected engine embarrassing, but it is also hard on the engine, the starter, and the battery.
One of the marks of a pilot who understands fuel-injection systems is his ability to quickly start a heat-soaked engine. Different procedures work for the two types of common general aviation fuel-injection systems. So let's look at the two systems.
Continental fuel-injection system
The Continental fuel-injection system continually delivers a metered fuel flow to each cylinder through individual fuel-injection nozzles. Fuel is delivered from the fuel tanks to the airplane fuel selector valve. The fuel then flows through the fuel strainer and electrically driven boost pump to an engine-driven fuel pump. The engine-driven pump output pressure varies directly with the engine rpm. Fuel under pressure from the engine-driven pump is called unmetered fuel. The amount of unmetered fuel delivered to the fuel control unit is always greater than what the engine will need.
The metered fuel, which the pilot has tailored to his flight needs with the throttle and mixture settings, is delivered from the fuel control unit to the manifold valve and then out to the fuel-injection nozzles through the small stainless steel fuel lines. The excess fuel is referred to as fuel vapor (although it's generally mainly liquid) and is routed from the fuel control unit through a separate hose to the fuel tank.
Often vapor lock will also occur in the fuel control unit; occasionally it will occur in the fuel pump. It's almost impossible to push vaporized fuel with a fuel pump—the pump is designed to push a liquid, not gaseous fumes. The result is an engine that's almost impossible to start using normal cold-engine procedures.
Continental hot-start tips
Short of waiting one to two hours for the temperatures under the cowl to cool enough that vapor formation is no longer a problem, what should the owners of Continental fuel-injected engines do to quickly start their engines and be on their way?
The first step is to attempt to get rid of the heat in the engine compartment while on the ground. Pointing the airplane into the wind and opening the oil filler door in the cowling will help the trapped heat on top of the engine to escape. If you are parked with the tail to the wind, opening the cowl flaps all the way and opening the oil cooler door will quicken cooling. Don't forget to close the oil filler door before attempting a start.
If you are only going to be on the ground for a short time, say less than 30 minutes, this may not do the trick. One technique that I first saw printed in a Cessna Pilots Association Technote is worthy of note.
This technique depends on circulating liquid fuel driven by the boost pump through the engine-driven fuel pump, the fuel control unit, and back to the airplane's fuel tank for two minutes before attempting a start. Remember that excess fuel is returned to the tank through the fuel vapor return line.
The theory here is that circulating liquid fuel through the engine-driven pump and fuel control unit, and returning it to the tank, will carry away the heat and vaporized fuel in the system—replacing it with cold liquid fuel that can be introduced into the cylinders for a successful start.
Follow these simple steps: Mixture all the way out like you're trying to shut off the engine, throttle all the way in, turn the boost pump on High—and leave it there for two minutes. It may sound like the boost pump is making a lot of noise but it won't hurt the pump. After two minutes push the mixture all the way in, and watch the fuel pressure needle. When it peaks, shut off the boost pump. Pull the throttle aft to the idle setting, turn the boost pump on Low, and turn the key to start.
The engine should start. It may be a little rough and you may need to momentarily switch the boost pump on High to purge any remaining vapor, but the engine should start within six blades of prop rotation. Lean for normal ground operation, and you are on your way. If it doesn't start, go through the fuel circulation process again—it's important to keep the fuel circulating for every second of the two minutes.
Precision Airmotive (Bendix) RSA fuel injection
Lycoming engines are equipped with Precision Airmotive ( nee Bendix) RSA fuel-injection systems. Since the RSA fuel-injection system does not have a provision to circulate the excess fuel and vapor back to the aircraft fuel tank, the Continental procedure outlined above won't work. The hot-start dilemma is further complicated by two facts. First, the amount of fuel metered to the engine in the RSA system depends upon the volume of airflow through the servo venturi and throat. During engine starts this airflow is insufficient to completely meter the proper amount of fuel.
Also, proper metering depends on there being liquid fuel in the servo. During heat-soaked operations, the fuel in the servo is often in a vaporous state and can't actuate the internal metering system correctly. Couple this with the heat-soaked fuel manifold and steel cylinder fuel delivery lines, and hot starts can be a real headache.
Cessna's new 172R
Cessna discovered very quickly that some remedial training and instruction for hot starting its Lycoming-powered 172R, 172S, and 182S models was in order and in early 1998 produced a videotape titled New Technology and Important Starting Procedures on the 172R.
The hot-start procedure in this video was to follow these steps: Throttle one-quarter inch in, mixture all the way out, master switch and auxiliary fuel pump On, clear the prop, and turn on the ignition switch. When the engine starts, release the ignition switch and push the mixture control all the way in to full rich; turn off the aux pump. This procedure works with mildly heat-soaked engines. For fully heat-soaked engines, a successful start will depend on using a procedure that gets the fuel/air mixture to a known point. In this procedure that point is when the engine has too much fuel, or is flooded. When an engine is flooded, it can be started predictably by adjusting the incoming airflow to achieve a mixture that's capable of supporting combustion.
Lycoming fuel-injection starting tips
The procedure that yields dependable results is to flood the engine with fuel by pushing the mixture and throttle all the way forward and turning on the boost pump for five to 10 seconds. Then turn the boost pump off and pull the mixture to off. Turn the key to crank the engine with the throttle all the way in (open). As the engine rotates, slowly pull the throttle back toward idle.
Closing the throttle gradually lessens the amount of air flowing to the cylinders—at some point the proper air/fuel mixture will be achieved and the engine will start to fire. When the engine starts to fire, continue to move the throttle toward idle, and be ready to gradually push the mixture control to full rich. During this procedure the engine will again start to stumble—this is the signal to push the mixture forward. The engine should then settle down into a normal idle.
If the airplane has a manual primer, it is often helpful to squirt a couple of shots of prime into the cylinders just before attempting a start. This procedure will bypass the fuel-injection system and squirt fuel into the cylinders. It may be necessary to feed fuel to the cylinders with slow application of the primer until the fuel-injection system clears itself of vapor and begins supplying fuel.
Manifold drains
Flooding the engine has a tendency to wash lubricating oil off the cylinder walls, so it's critical that the intake manifold drains are open. These should be checked at every annual.
These drains, which are also installed in Continental engine induction systems, are essential to safe engine operation. If the drains are blocked, the liquid fuel pumped into the engine during flooding can enter the combustion chambers and result in a hydraulic lock, which will severely damage the engine. Ask your mechanic to point out these drains for you.
Share your results
Fuel-injection systems for light aircraft eliminate carburetor icing, meter fuel evenly, and are extremely dependable. These systems provide trouble-free service only as long as the owner understands the causes of hot-start problems and develops a procedure for dealing with them. Experiment with your engine, share your hot-start techniques with other owners, and develop a method that works for you.
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