The Science of Starting a Piston Aircraft Engine

We pilots bring to our flying different levels of understanding of things mechanical and electrical.  Some of us can read an electrical schematic with our morning coffee.  Some of us are a menace with a torque wrench.   Some of us have spent years maintaining, even building, airplanes.  Some of us, sadly, know little of the machines we fly.  We, as members of the Wings of Carolina Flying Club, are blessed to know James Garlick, A&P and IA, who not only manages the maintenance of our fleet, which flies approximately five thousand hours every year, but also educates the rest of us in the mysteries of aircraft maintenance, systems, and operation. 

If you fly long enough, you will eventually have that day when you can’t get the dang thing started.  Maybe it’s a carbureted engine on a cold day.  Maybe it’s a fuel-injected engine on a hot day.  Perhaps you will go out on the internet and search for advice on starting aircraft engines.  And you will find some very peculiar ideas, mostly the product of anecdotal evidence unsubstantiated by science. We asked James to take us back to first principles and help us understand the engineering and chemistry behind the combustion process in aircraft engines.                  

-George

Wondering why it is so hard to start an airplane when it’s cold outside?  Suspect that there is a secret procedure that works every time? And you probably suspect, too, that we are withholding that secret from you. I wish that were true.  There is no secret. There is only science. And from it we can extrapolate some basic principles

The joys of being a general aviation mechanic in the colder months are few and far between. All the airplanes seem ornery, and as a result, so do the mechanics. One of the things that we actually do enjoy during the winter months is witnessing the successful cold start of a carbureted airplane. The starter will live to see another day, the battery is no worse for wear, and the air filter isn’t on fire… it’s the little things in life. So how does a mere mortal figure out how to start one of these confounded, aggravating, stubborn machines? Honestly, I wish it there was a simple answer. But what I do know is that there are techniques that work more often than not, and my hope is that sharing those techniques, along with the theory that backs them up, will make starting our little prop-driven buckets of joy a little bit easier. For the sake of simplicity, I will focus on carbureted planes, but the theory is the same regardless.

Let’s start with the theory. Actually, it’s proven science. It has been proven time and time again that when you have the proper mixture (recognize that term?) of a hydrocarbon (avgas) and oxygen (you know, air) combined with an ignition source, a combustion reaction results. This is simply a fancy way of saying that if you have fuel, air, and spark, things go boom. The hardest part about facilitating this process is finding the proper mixture.

The stoichiometric (fancy word for ideal) ratio for avgas is 14.5 to 1. This means that one pound of avgas combusts most efficiently when mixed with 14.5lbs of air. While starting an airplane, our target range is much, much wider than that. Even still, this fairly large target can be a tough one to hit. If we don’t add enough fuel, it’s too lean. If we add too much, we’ve flooded the engine. Either way, the mixture is far enough away from stoichiometric where it simply won’t burn with the heat available from the spark plugs. In the ratio described above, the units of measure are pounds. It can be ounces, grams, graves, or any measurement of weight. (Technically, mass, but forgive me the technicality here.) So, all we have to do is weigh the air inside the engine, compare it to the weight of the air it will be ingesting when we attempt a start, and add the right amount of fuel. (Just to get super nerdy for a second, on a standard day, in a Cessna 152 with an O-235, there is approximately 0.0109747965lbs of air in the engine. This means we need 0.00078391lbs of fuel to achieve an ideal mixture.)

Easy, right?

Except, not only is there no good way of measuring these values in real time, the values don’t stay constant, either. Avgas stays constant enough by itself, but there are things that we can do to disguise exactly how much fuel we are introducing into the engine. More on that later. Then there’s the air. As pilots, we understand that the air we get to play in when we are lucky enough to get an airplane started is affected by many different factors. Pressure, temperature and moisture content all play a role in the weight of a given volume of air. If we stop to think about the significant effect that these variables have on other aspects of flight, (TAS, density altitude performance, etc.) we start to get a sense of just what these changes mean on the small scale, minute, really, of a small piston engine starting. What would .00078 pounds of fuel look like?

Now that your heads hurt and you’ve checked to see if any of your scales go that low, let’s discuss how we utilize this knowledge in a practical way.

When starting an airplane, it’s much easier to add more fuel than it is to get rid of it.

For this reason, we want to approach the air/fuel mixture from the lean side. On a cold day with a plane that hasn’t flown in several hours, you are starting on the very, very lean side. So we need to add fuel. The right amount of fuel.

There are a number of ways to do this, but some are more effective and safer than others. In carbureted aircraft, we have primers, and they are the preferred way of adding fuel. Primers are simply a way of introducing fuel to the engine without using the carburetor. The fuel is pushed out of a small barrel with a piston, through a series of small lines and deposited just behind the intake valve in the engine, right where it needs to be. The technique is fairly simple. I generally like to try and start the plane without any prime, just to see if I get lucky. You never know, right? Actually, I am doing something useful – I am confirming my hypothesis that this engine on this day will be too lean to start without priming.  And I am learning what to do next. When this fails to get it started, simply give it a shot of prime and try again. Continue until the engine starts. It’s just that simple. Sort of….

Sometimes, you may need two shots of prime prior to engaging the starter. Generally speaking, the third shot of prime is simply replacing the first that has had enough time to run down the intake tubes, and is considered ineffective – and hazardous.  There are always exceptions, and there is no hard and fast rule that applies to all aircraft, although I would argue that priming less rather than more and limiting cranking time are universally applicable principles.  Figuring it out is just part of learning about the particular planes you fly. To further confuse things, I mentioned before that there are ways of disguising how much useful fuel is actually getting into the engine, and use of the primer is where this can creep up. If the primer is pulled out, then immediately shoved back in, chances are, you didn’t get as much fuel as you should have to the engine. The barrel of the primer needs to fill up with fuel before it gets pushed in, and this can take a second or two. You should feel the resistance of the fuel being pushed through the lines and into the engine. While a slight hesitation in pushing the primer back in can be beneficial, the opposite is true when it comes to the transition from the primer knob to the key to engage the starter. As soon as the primer is in, you should immediately engage the starter. This means you will have already cleared the area and ensured that a start attempt is safe before the priming takes place. The longer the delay from prime to crank, the more fuel has puddled in places other than where we want it. This is exactly what happens when we prime too much. We go way beyond the amount of fuel that we need, flooding the engine.

There is another popular method for introducing fuel to an engine for startup – pumping the throttle. While this method can be effective, it is less than ideal for a number of reasons. Most carbureted aircraft engines utilize updraft carburetors. This simply means that the carburetor sits on the bottom of the engine, and the air flowing into the engine creates an “updraft” as it passes through, carrying fuel with it along the way. It works incredibly well, and in the event of a fuel leak, the fuel doesn’t leak onto a very hot engine, reducing the risk of fire. But without the pressure differential and resulting updraft when the engine is turning, it is a pretty ineffective way of getting fuel to the combustion chamber, located 18 inches above. Pumping the throttle actuates the accelerator pump in the carburetor. The accelerator pump is not meant nor designed to be a means of priming an engine.  It has only this in common with the priming system:  it moves fuel vaguely in the direction of the engine.  The accelerator pump is actually a part of the carburetor. So when it is actuated, it pumps a significant amount of fuel into the throat of the carburetor, where the vast majority of it simply falls into the carb heat box. The accelerator pump is actually a part of the carburetor. So when it is actuated, it pumps a significant amount of fuel into the throat of the carburetor, where the vast majority of it simply falls into the carb heat box in liquid form. The little bit that does get atomized and carried up to the cylinders with the airflow created by cranking the engine can sometimes, almost by accident, be enough to achieve the desired mixture and start the engine. Or, it could be just off enough to cause a backfire, where the combustion process happens while the intake valve is open, causing a ball of fire to work its way back down the intake pipe, through the carburetor, and into the now fuel soaked carb heat box. We can all see how this is less than desirable.

The primer is akin to drinking water through a straw.  The accelerator pump is akin to trying to drink water by pouring it on your head.

Now, while we now know that we should use the primer as opposed to the throttle to introduce fuel that is not to say that we should think of the throttle as having to remain stationary during startup. In fact, moving the throttle slowly through a limited range close to idle, we can manipulate the airflow enough to have an effect on the mixture in the combustion chamber, giving us an even better shot at a successful start.  Remember, the throttle is principally an air valve, not a gas pedal.   Very often, slowly closing the throttle will induce a start by limiting the airflow and therefore enrichening the starting mixture.

Now you have some solid science, and a healthy dose of sarcasm, to bear on the problem. It is important to remember that every airplane will be slightly different. Each make and model, and each specific airplane, will have enough idiosyncrasies to make a significant impact on starting characteristics. Slightly blocked primer nozzles, spark plug wear, magneto timing drift, to name a few, will all necessitate adjustments in actions to achieve a successful start. Airplanes may differ, but chemical reactions, thermodynamics and machines are all governed by known principals. The more we understand them, the more effectively we can use them, and our chances for success increase dramatically.