Technical question for the technical people

[marvin347]

I've had these questions mulling in my head for sometime, but need a way to explain them better to my students. I'm an instructor and a bit embarrassed to bring this up at work. Mechanics were never my strong point.

1) Why exactly do you need prime for a cold start and not for a hot? My understanding is that it injects fuel into the cylinder directly, but why does the engine temperature matter?

2) (Before Starting the aircraft) What exactly is happening when the throttle is being pumped, compared to just leaving it at full throttle (Both for Carb and Fuel injected). My understanding is that fuel is placed at the mouth of the carb, but don't understand the process of how it is getting there, compared to throttle open, compared to what happens in a fuel injected.

Thanks in advance

[paco]

A hot engine will vaporise the fuel better - it's the vapour that ignites.

Not quite sure what you mean by 2), but the throttle is attached to an accelerator pump which is supposed to provide a small squirt of fuel to prevent a weak cut when the throttle is opened quickly. This can also be a crude form of priming

All this is explained in my PPL books (see Pilot Training & Ground Studies - Electrocution's Aviation Books (http://www.electrocution.com/aviation))

Phil

[NutLoose]

Hi marvin347

Not a prob it's good to ask. this will help regards priming, page 48 onwards

http://www.lycoming.textron.com/support/tips-advice/key-reprints/pdfs/Key%20Operations.pdf

Some more useful guides are here

Key Reprints - Lycoming (http://www.lycoming.textron.com/support/tips-advice/key-reprints/index.html)

The Accelerator pump on the carb is as described above, it is on the side and integeral with the carb, it basically squirts fuel out of the carb through a jet as a mist and into the induction side of the engine above the carb, pumping this as a primer is a bit of a no no and bad practice, the reason being if you get a backfire it can ignite the fuel in the carb and set fire to the airbox and filter, in this case you crank it until it has been drawn through the engine, "better in than out as we say" after all that is where the burning fuel was supposed to have ended up in the first place. :)

The primer nozzles do not squirt fuel into the cylinder but into the cylinder head just behind the induction valve as a mist, allowing you to have an charge ready to burn when drawn into the cylinder, read the above link for more ref cold and hot running.......

I find a lot of instructors that know nothing about what is happening in front of them sad to say, nice to see someone asking :).

[172_driver]

The examiner on my CFII x-ride 'pumping' the throttle on my fuel injected C172... made me chuckle :)

[Dodo56]

Cold air is more dense than hot air (it's why a balloon rises) thus contains more oxygen. To keep the fuel/air ratio within the limits where it will burn in a cold engine (about 15 to 1 air to fuel by weight) you therefore need to introduce more fuel.

Air-fuel ratio - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Air-fuel_ratio)

[Short_Circuit]

Also, when cranking the engine, the fuel is not being drawn into the venturi at normal rate and does not mist as well as it does during normal running RPM. The pumping of the accelerator pump adds extra supply of air/fuel mist into the throat of the carb which will easier ignite during start.

[mini]

Marvin,

Prime (adding extra fuel) for a cold start is best explained as compensating for condensation losses on the manifold/cylinder head, ie some of the fuel doesn’t reach the cylinders. Its easily understood & remembered. When everything gets up to working temp, condensation doesn’t happen. (many other factors involved, but this explanation works 999/1000)

Pumping the throttle injects petrol/gas directly into the inlet manifold side of the carburettor, under normal operation, petrol/gas is drawn into the manifold, via the carb and through various jets etc by demand expressed as vacuum, mm/Hg.

When you snap open the throttle, you want an immediate increase in engine rpm, this needs a corresponding increase in fuel, the standard jet set up can’t do this so we have a device called an acceleration pump, basically a syringe that squirts petrol/gas into the manifold when you snap the throttle.

I haven’t explored the links offered by previous posters but I’m sure they cover this issue… carbs 101 for 1st year Motor Mechanics…

[marvin347]

Thanks to all who have responded. I now feel that I can explain these processes much better to my future students.