Anyone know as to why fuel injected engines tend to be more difficult in starting when using a hot start procedure? I know it has to do with the fuel already in the engine and/or fuel vapors. I've been curious about this for quite some time but have never heard an in depth explanation as to why.
George Braly, inventor of the Gamijectors fuel injection nozzles, has a slightly different take on the vapor lock. Here is his recommendation and explanation as related by John Deakin:
If anyone consistently has trouble starting any of these flat TCM engines (turbo'd or not) when they are cold, they are either doing something seriously wrong, or something in the engine is not right. Throttle cracked, mixture rich, a shot of boost (or prime) until the fuel pressure steadies, or about five seconds, and the engine should start and run on the first turn of the prop.
Hot starts are equally easy.
Yup, that's what I said, there's NO difference between a cold start and a hot start, PROVIDED you do it using science, not folklore. In fact, hot starts are easier. Everyone has a favorite method, and some of them even work on some airplanes, some of the time. Most often, you will hear of flooding the engine deliberately, then cranking with some combination of throttle and mixture settings until the engine starts. Generally, that's NOT a good idea, because most methods of doing this end up with copious quantities of raw fuel in various unknown locations, creating a fire hazard. Fuel on the ground can catch fire and cause the loss of the airplane, fuel in the exhaust manifold can "torch" and sear the paint above the exhaust pipes, and fuel in the induction manifold can do great damage to the tubing, much of which is not metal. Yes, many get away with doing this, some for years. But I've seen all three results, and in each case the pilots whined, "I did it the way I always do it!"
There is a bit more science involved with hot starts, because the heat developed by an engine that has just been shut down can have an undesirable effect. The key here is to understand that with the engine running just before shutdown, cool fuel is circulating through all the fuel lines and the engine-driven fuel pump, effectively keeping the plumbing AND THE PUMP cool from the inside. There is also some cooling airflow, blowing hot air out of the accessory compartment. Once you shut down and trap fuel in the lines, the large thermal mass of the engine transmits the heat throughout all metal parts of the engine and the engine compartment. This "cooks" the fuel in the lines, and more importantly, the heat from the engine "soaks" into the engine-driven fuel pump, warming that fuel. Since there is no cooling airflow to partially cool the outside of the fuel pump, the temperature of that metal assembly will actually rise after shutdown. The fuel trapped in the fuel pump heats up, slowly develops bubbles, and the engine-driven fuel pump becomes full of them after about 15 minutes.
One thing every new pilot on the old radials learned was that engine-driven fuel pumps don't work very well with mixed air (vapor) and fuel, they cavitate. On the other hand, electric boost pumps are quite happy to pump either fuel or air, or both. The two complement each other, with the additional benefit of having a backup if the engine-driven pump takes a break.
If you prime the hot engine, and turn the boost pump off, you'll probably get some fuel and air bubbles through to the cylinders. The fuel you injected into the engine is enough to make it cough to life, perhaps even run for a second or two. But as soon as that fuel and the little bit of fuel in the injector lines is used up, the engine-driven fuel pump is simply spinning its wheels in vapor, unable to move the needed fuel, and the engine dies. Repeat that, and the same cycle occurs until the battery goes dead.
Yes, you can run the electric boost pump for the start, but it's so good at moving fuel (and air), even with bubbles in it, it's very hard to control the actual flow, and the mixture will probably become too rich to run. Some master the trick of running the boost pump, and slowly moving the mixture control or the throttle until a viable mixture exists, but it's a difficult trick, and different for every engine. Some push the throttle in and run the boost pump before hitting the starter, but this pumps raw fuel into the engine, and possibly overboard, creating a very real risk of fire. Just because you've never seen one, don't think this cannot happen! Even if you get the engine started, the fire can do damage, scorching paint at best, and burning up the airplane at worst.
A better starting method is needed, and there is one, at least for the TCM engines. We haven't had much chance to try it on the Lycomings.
For starts within the first 10 or 15 minutes after shut-down, just put the mixture rich, crack the throttle, DON'T prime, or just a tiny shot, and go. If that fizzles once, don't run your battery down with further attempts, just go right into a hot start.
Simply leave the mixture in full lean ("idle cutoff"), and run the electric boost for one full minute. Sixty seconds. Time it, by the clock. More won't hurt a bit, but less may well not be enough. A full 60 seconds, not a second less. The first few times you do it, it will seem interminable, but there's no reason you have to just sit there. When you know you're going to use this procedure, flip the pump on early, hit the stopwatch, and go ahead and do your cockpit setup, or study the instrument departure, or brief your passengers (you DO brief passengers, don't you?)
This little trick uses the ELECTRIC pump to pressurize the lines to the engine pump and the chamber inside the pump case itself. Since that fuel can't go beyond the fuel control with the mixture shut off, the only way out is through the rather small "vapor vent return" line. This is the exact purpose for which this line is provided.
Once this "cooling" step is done, the start is identical to the cold start, and just as easy. At the end of the sixty seconds, let the electric pump continue to run while you push the mixture in until the fuel flow stabilizes (as for the cold start), flip the boost off, and hit the starter. Instant gratification. Well, sixty-second gratification, anyway. In effect, this procedure converts a hot start into a cold start. (Actually, it may be better than a cold start, because the engine is warm, and the fuel will vaporize better.)
What's the magic behind 60 seconds? Well, if you lay your hand on the fuel pump of a recently shutdown engine — ouch, you'll burn yourself. Run the boost pump, and you still won't be able to touch the fuel pump until about 60 seconds have gone by, and then you'll actually feel the case cool off from the fuel running through it. There isn't a lot of fuel passing through it, because the vapor vent return line is fairly small. But new, cool fuel IS coming in, and it IS driving out those nasty little air bubbles, and it is cooling the pump from the inside out. Patience, take the full 60 seconds. Works great.
I have heard people complain that this longish use of the boost pump will run the battery down. Maybe, but so will repeated attempts to start a hot engine using any other way, and the starter is a LOT more load than the electric boost pump! Also, your battery is an important reserve of electrical power, and if it cannot perform this function briskly, then I think the airplane is unsafe for any flight beyond a simple hop around the pattern in good weather. Remember, the engine is hot, implying a very recent flight, and that battery should be fully charged.
You may have your own way of doing the hot start. It may work for you, in your airplane, most of the time. That's fine, use it. But the method above works in ALL these TCM fuel-injected, flat-engined airplanes, ALL the time. It has the advantage of leaving the engine in a known condition, fully equivalent to the cold engine. I may not have seen all the methods out there, but I'll bet I haven't missed many. I like this one best of all, for it uses science instead of oaths and imprecations. Credit George Braly with this one, not me.
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