To prime or not to prime

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TAILspin38
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To prime or not to prime

Post by TAILspin38 »

Did a x/c flight last night and landed at a new airport to take on fuel. After having been shut down for approximately 30min.(guess) as I went to start the plane I was not sure if I should use fuel pump for warm engine start (unfortunately I did not look at the oil temp).I primed for 2sec, the plane started but at the very end of my crank cycle. My question is does anyone use oil temp as a guide to use or not use a prime for hot/warm start? Thanks
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Steve
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Re: To prime or not to prime

Post by Steve »

I usually prime for 2 seconds for a warm start, 4 or 5 seconds for a cold one. I use an oil temperature of about 110 degrees as the dividing line, although I also look at the OAT when deciding which start method to use. I don't have any supporting documentation for this routine, just that it seems to work well for my particular engine in the environment of the Southwest US.
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Rich
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Re: To prime or not to prime

Post by Rich »

For the DA40, a way to assess how "warm" the engine is involves checking:

-Oil temperature gauge. Recognize that if the engine hasn't run for a bit what you're really seeing is more proxy for the rear of the crankcase. This is good information.
- CHTs. You'll get valid indications shortly after you hit the master. This will give you an idea how much heat has been retained in the cooling plenum.

Starting these engines is kind of a black art and, as Steve implies, can vary from one engine to another. Probably the main reason for priming is to get liquid fuel into all the fuel delivery components. After days sitting in the hangar there will be lots of voids in these lines. If the engine was running just a few minutes ago, they're already probably full enough. But if it's really hot in the engine compartment fuel will boil off in parts of the system and now you have voids - i.e. possible vapor lock.

Note that what kind of priming actually happens is a function of mixture and throttle position.

With the Surefly I do less cold-engine priming - a count of 3 - with mixture and throttle full forward. Move throttle back to a high-idle position (or else :shock: ). For warm starts I don't prime at all. I'll crank with mixture full lean and use my mystical powers of detection to assess engine response to determine when to move it forward.
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Charles
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Re: To prime or not to prime

Post by Charles »

I always try to start without priming if I suspect a warm engine condition, which I determine by simply touching the cowling — I assume a warm engine if I can feel any heat. My warm start method involves opening the throttle halfway or more, cranking, and when it starts firing, retarding the throttle and gradually increasing mixture. If the engine won’t fire within 5 seconds, then I will prime and try the cold start method. Works pretty much every time.

I had issues with warm start during a few months but that turned out to have been due to a defective Slick Start.
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Don
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Re: To prime or not to prime

Post by Don »

I have had four factory new Diamond Stars since 2003 and found each plane seemed to have its own personality when doing hot starts. With each aircraft, I had to experiment to find the sweet spot. My wife and I try to fly at least once a week and usually stop at an airport that has an on-field restaurant to grab some lunch. After about an hour or so, we return to the plane which makes for a hot start. First thing I check is the oil temperature gage which normally will be in the low 140's F. Before cranking the engine, I then open up the fuel mixture and advance the throttle fully open and fully back two times. Then, after advancing the fuel mixture again, I open the throttle about 3/4" and hit the fuel pump for just less than a second for a short squirt. When cranking the engine, I slowly bring the mixture up and engine starts right away. That said, results for other Diamond's may vary.
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Lou
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Re: To prime or not to prime

Post by Lou »

There’s a funny line in a recent Paul Bertorelli video on AV Web something like “There’s a procedure to hot start the engine that works every time but nobody knows what is” :D

Put me in that camp. I use the throttle max, then open a crack, start cranking and advance mixture until it catches technique. Works 80% of the time. The worst is if I start up and idle over to the fuel pumps, and the engine is not quite hot. Don’t understand why it is so finicky but I usually get it going.
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TAILspin38
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Re: To prime or not to prime

Post by TAILspin38 »

Wow! Sounds like using your special TECHNIQUE to start your trusty old standard pickup truck. Thanks for all the valuable insight. I will definitely try some of these signature techniques :bow
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Re: To prime or not to prime

Post by CBeak »

Good to know that my special “play around with it until it starts” method is pretty much the norm😁
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dant
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Re: To prime or not to prime

Post by dant »

I scale my priming down based on how warm, from 4 seconds at <50 to zero at 110+ or so. Priming is always full throttle, fuel pump on, mixture full forward and time it such that it's full off at the end of the time.

When priming, then I put the throttle at around the top of the E, pump on, crank until catches, slowly advance mixture.

When not priming, then throttle is open a bit more, then same deal. I haven't found the right spot yet. Not open enough and it floods easily. Open too much and even the brakes can't keep you still when it catches. I talked to my mechanic about it and he apparently hot starts with WOT and just has enough experience that he recognizes when it's about to start and run away and pulls the throttle back at the right moment.
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Charles
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Re: To prime or not to prime

Post by Charles »

Lou wrote: Wed Jan 26, 2022 2:30 pmThe worst is if I start up and idle over to the fuel pumps, and the engine is not quite hot. Don’t understand why it is so finicky but I usually get it going.
Same here, until I started applying the warm start technique I described earlier even with the engine almost cold: open throttle to 50+%, idle cutoff mixture, crank until it fires, retard throttle while increasing mixture. The engine will fire within 5 seconds of cranking. Works every time.
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