engine priming

[Schmay]

I'd like to hear the opinions out there on the proper way to prime an engine. I always taught my students to only use the primer...not to pump the throttle. I remember reading a long time ago that pumping the throttle can cause a fire. I've also seen a student set fire to a Warrior.
But yet, I still have people come in for flight reviews, advanced training, etc. who pump the throttle for priming.
Am I wrong in having them only use the primer? It seems like the primer works better anyway (at least from my experience).
I'd like to hear your thoughts on this matter.

 

[Vik]

I prime and I will teach my students to prime.

If you pump the throttle, it is a fire hazard. I don't know why people teach this method. If the manufacturer wanted you to pump the throttle, they would put it in the POH. Also, why provide a primer if they want you to pump the throttle?

 

[avbug]

Priming an engine is the act of preparing the engine to fire. Most engines have some form of dedicated primer, but some do not. Priming systems in horizontally opposed flat recips generally deliver fuel directly to the cylinders, placing a charge of raw, unburned fuel directly in the combustion chamber.

Only a very small amount of fuel is necessary, and directly priming an engine can cause an afterfire, which comes out the exhaust stack. One should prime only the bare minimum necessary (this doesn't apply to radial engines, which generally take a lot of prime through the supercharger when cold, and will probably catch fire anyway).

Pumping the throttle is using the accelerator pump, and this only works on engines equipped with an accelerator pump. Many airplanes don't have one, and it's quite comical to watch a pilot pumping the throttle out of habit when he or she doesn't understnad the system, or know that no accelerator pump is on the carburetor.

Watching someone pump the throttle on a fuel injected engine is a lot of fun. Usually when asked, the response is, "my instructor taught me to do it." Apparently this excuses the lack of ability to think for one's self, but I digress.

Use of the accelerator pump places a very small squirt of fuel into the induction manifold. In most light airplanes, the carburetor is an updraft arrangement, and the fuel squirts up...and runs back down through the carburetor and into the carb air box. It collects there, ready for a backfire, which is the proper term for an induction fire (a fire in the exhaust being an "after-fire). The carb air box has a drain hole, which permits the fuel to run into the cowling, where it cannot be contained once ignited...until it's burned itself out.

If one intends to use the accelerator pump during start, one should pump during the start However, this places the throttle plate in a position to be opening fully and closing fully repeatedly, while raw fuel is sprayed into the induction. The liklihood or potential for a backfire increases dramatically as the mixture changes back and forth; the potential exits to set fire to the carburetor and air box, as well as do damage to the induction by a backfire.

If one is to use the accelerator pump at all, it should be a couple of quick pumps before starting, and perhaps one as the engine turns. The throttle should remain closed during the start itself. The accelerator pump is not a priming tool; it's purpose is to enrich the mixture briefly during a power increase to make a smooth power transition. Nothing more. It's there as part of the automatic function of the carburetor, not as a tool to get the airplane started. Let the idle jet do it's job.

One may also prime some fuel injected airplanes with an auxilliary fuel pump (boost pump). This will work on some airplanes, but not on others. Again, know the system. On some airplanes, pushing in the mixture and applying boost will do nothing more than apply pressurized fuel to the controller. Until the engine begins turning, nothing more happens, and openign the throttle does nothing more than open the air valve butterfly, or the throttle plate. On others, one can put fuel directly to the cylinders via the fuel injection nozzles, but can easily flood the engine.

One needs also to pay attention to the damage done during priming and the subsequent start. Fuel is a solvent, and once placed in the cylinder in liquid form, it quickly removes the protective lubricant from the cylinder walls during start. The result is an increase in cylinder wear by an order of magnitude. (or two). Don't overprime, and don't prime when not necessary.

Radials don't see the common use they once did, but you may occasionaly see one if doing a checkout in something like a Cessna 195. Overpriming a radial engine, aside from fire issues, has the very real hazard of hydraulic lock in the bottom cylinder. Liquid lock, or hydraulic lock, is usually the result of trapped oil in the lower cylinders. However, the same thing can occur during start by overpriming some engines, and in the event of a valve malfunction, or a fuel flow high enough to preclude being drained through the valves fast enough, the result can be a bent slave connecting rod (and subsequent engine failure).

In the winter, a radial often needs a lot of prime, through the supercharger, to get a good start. The liklihood of a fire goes up exponentially, and one must be aware and prepared. For most radial supercharged engines, the fuel dumps directly into the supercharger from the carb, or from an aux boost system, and doesn't go right to the cylinders. Accordingly, the cylinders still aren't getting a raw fuel mixture. It's not uncommon, however, to fill the supercharger 1/3 full of avgas during the start when it's really cold...leading to the issue of the supercharger drain creating a massive fire hazard if you elect to discontinue the start. (The C-97 was famous for induction fires as the fuel would pool in the inside of the airscoop, and the drain hole would invariably be plugged with congealed oil).

Prime where necessary, but only where necessary. If you are in the habit of pulling the propeller through in the winter (not a good idea), priming first will only dramatically increase cylinder wear and reduce any available residual lubrication on the cylinder walls and valve stems. I see a lot of people prime, then pull the prop through; it accomplishes nothing but damage.

One final controversial note: after-shutdown priming. When operating a fuel injected engine to remote locations, I made a habit of opening the oil filler and check doors on airplanes such as the Cessna 207, to act as an additional vent to cool the engine compartment. In order to facilitate start-up again, I often flooded the injector lines using the boost pump after shutdown. There are those that will debate the safety and efficacy of this technique, but I still practice it.

If you do it, however, be aware that you've just placed an unburned charge of fuel in the cylinder after shutdown. There are obvious safety implications. In the back country, I felt I could mitigate these implications and gaurd the airplane without fear of someone inadvertantly moving the prop or causing the engine to fire (it can fire on it's own without outside intervention, so bear that in mind). I felt and do feel that given the alternative of being stuck in the event of a hard start, anything I could do to assist this was to my benifit. It's not something I practice with a student (on the rare occasion I get a chance to instruct, any more), but it's still a viable practice under the right circumstances. Let the flames begin.

 

[A Squared]

Avbug,

You wrote:

".....placing a charge of raw, unburned fuel directly in the combustion chamber....."

I'm sure that you've had more dissasembled cylinders in your hands than have I, but I've had my share. I have never seen an aircraft cylinder in which the primer or fuel injection nozzle places fuel directly into the combustion chanber. In all the cylinders I have seen, the primer nozzle screws into a threaded boss in the cylinder casting, but the nozzle actually discharges into the intake port, upstream of the intake valve, not into the combustion chamber at all. Do you know of any engines where the primer or injectors actually discharges into the combustion chamber? If so, what models?


Regarding the run-back and pooling of fuel:

Many carburated G/A engines (mostly continentals) have primers which introduce fuel into the intake manifold a long way from the cylinders. I know that on the O470-R, the primer nozzle discharges into the vertical portion of the intake manifold, directly above the carburator. I beleive that O-200's are this way also. With this type of arrangement, fuel from the primer is no less likely to run back to the air box than fuel from the accelerator pump.

regards

 

[aggiepilot87]

Do you know of any engines where the primer or injectors actually discharges into the combustion chamber? If so, what models?

Diesel !

I know that on the O470-R, the primer nozzle discharges into the vertical portion of the intake manifold, directly above the carburator. I beleive that O-200's are this way also.

C-90 (about the same as O-200) injects prime into the manifold above the carb.

 

[Vik]

avbug,

Will you adopt me?

 

[avbug]

A Squared,

You're correct in stating that the fuel is dumped into the intake plenum in the cylinder head for most priming arrangements. A few do install the fuel diretly into the manifold, though most are plumbed right into the cylinder head.

I should have been more clear.

My point was that raw fuel gets put directly into the cylinder in a liquid form with the opening of the valve, and thus the lubricant coatig on the cylinder walls is destroyed prior to start.

For engines using boosted injector fuel for prestart priming functions, the same thing occurs.

Rather than look at the specific model of the engine (ie, O-470-R, one needs to look at the installation. The arrangement of priming lines may be different from one installation to another. This is partially because on many aircraft, the priming system isn't a powerplant item, but a customer-supplied airframe item.

On some installations only three or more cylinders get primed. On the 0-470-I on the Cessna 182, for example, the rear right cylinder is generally tapped for manifold pressure at the same fitting point used for a primer line; this is in the intake plenum prior to entry into the cylinder, as you described (but not in the intake manifold). On other such installations, the manifold pressure sense line is more commonly run to a common port on the intake manifold, while all cylinders end up getting served with the primer.

I should have included in prior comments the fact that under some circumstances, failure to lock in the primer during takeoff or other phases of fligtht can result in a rough engine or engine failure if the primer backs out. Likewise, and more likely and common, is a flooding situation with a partially closed primer pump.

 

[bobbysamd]

Priming

Any votes for looking up the correct procedure in the airplane's POH?

I've seen Seminole pilots prime with the primer and by giving it one stroke on the throttle because the airplane has throttle pumps. We had a great deal of disagreement at Riddle on the correct procedure for priming a Seminole. However, if we shut one down in flight and if cracking the throttle didn't work for restart, giving it the one stroke of throttle would work.

Having said all that, I'd second Schmay's opinion. Use the primer. Although his BFR people might be using throttle to prime, I feel they are wrong. One of my favorites are the folks who stroke away with throttle and wind up with a puddle of fuel on the ramp.

 

[starchkr]

On the note of backfires and overpriming the engines...

Case in point as to why you want to avoid a backfire from overpriming.

Last week, Wednesday i believe we had a BE-55 ready to start up here in Jackson. The pilot primed and started his left engine only to cause a backfire which for some reason started a fire under the outboard portion of the wing from the engine out. Somehow the heat and flame reached what had been a slow undetectable leak from the outside and blew his wing apart. Observations after the incident noted no visible stains from fuel under the wing, yet it was there. Now the second part of why it is believed that the fire started in the first place. From observations and mechanics ideas it is thought he might have had a leak in his fuel pump which dropped fuel into his cowl which is what caught fire under his wing from the backfire.

To make this all fit into context, backfires are bad, not only for the engine, but possibly even for your health. Priming is something that should be followed specifically in your POH, and not by what someone suggests you do just because it works for him. Anyway, blah blah blah... Do what is in print, and it can help to avoid problems like above.