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Fuel injectors - do they draw in ambient air?

10 Posts

I have come across this article

It basically says that the injectors mix the fuel with the air, which implies that air enters via some gap in the side of the injector.

Is that really true?

Administrator
Shoreham EGKA, United Kingdom

It doesn’t imply that at all. Injectors squirt a jet of atomised fuel into the incoming air stream.

Forever learning
EGTB

My understanding is they draw in ambient air for a NA injected engine and for a turbocharged engine they draw in pressurized deck air. I also understand it is used to aid in the atomization of the fuel. The primary air for combustion comes to the cylinder thru the intake air plenum and is mixed with the fuel mixture in the cylinder intake chamber above the intake valve. If you examine a fuel injector for a TCM engine, you will see a small screen where the air is sucked in.

KUZA

I wonder if that air inlet is responsible for many unexplained and possibly transient problems.

On cylinders 1 and 2 you will get a lot of rainwater getting in.

It also gives a whole new meaning to the issue of getting injectors cleaned. I think it was Mike Busch who wrote this is complete nonsense because they are constantly cleaned during operation with a high pressure solvent (avgas). I would think he would know about this…

Administrator
Shoreham EGKA, United Kingdom

Look at page 10 here.

It’s an excellent document. The fuel injection is a rather cool device, but to call it fuel injection is a bit misleading IMO. It’s more like a “carburetor version 2.0” or something. All it does is to divide the fuel to each cylinder instead of at the inlet of the manifold. You achieve better distribution of fuel, but no better atomization like in a real high pressure injection system. A normal carb is probably better for atomization.

Sorry, page 17, figure 10.

Last Edited by LeSving at 01 Mar 18:03

Interesting… that manual is up here already – local copy

Page 17 does confirm there is an air bleed

Can the owner of a TIO-540 confirm that their injectors have a little side pipe going into them?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Can the owner of a TIO-540 confirm that their injectors have a little side pipe going into them?

Both Conti and Lyco Turbo-Supercharged engines have an upper deck pressure pipe on each & every injector.

Last Edited by at 01 Mar 18:14
FAA A&P/IA
LFPN

Peter wrote:

On cylinders 1 and 2 you will get a lot of rainwater getting in.

Peter,

I seriously doubt that. I assume that #1 and #2 are the forward cylinders on your engine, With my TCM IO520BA, they are in reverse order, the even are on the pilot side, numbered from the front to rear as 6, 4, 2 and on the RH side the odd cylinders are 5, 3, 1. The injectors are well below the fins and down near the spark plug, well protected.

The cylinders that seem to cause most issues on the Bonanza are the two in the rear, 1 and 2. If there ever was a shock cooling effect, which I believe is an OWT, imagine the shock cooling when an airplane flies into a rain squawl and hits those front cylinders. You would thing that such a rapid cool down would affect the front cylinders more that the rear ones.

KUZA

This is my #2 cylinder and injector (bad photo, from 2002)

The #1 cylinder injector is however well out of the way, behind the cylinder (front of the aircraft is on the left)

From 2009:

Regarding the shock cooling due to water (rain) I don’t think rain is anywhere near dense enough. I am sure there is data on the internet but I would bet that even in the heaviest rain at least 99.9% of the space is just air. Well, on the Isle of Man it might be 99.8%

Administrator
Shoreham EGKA, United Kingdom

As pointed out in the Air Precision Bendix FI Training Manual (posted above), the amount of air that is actually drawn in is rather small and of little impact at normal power settings. It is greater at idle and very low MP settings since the pressure differential is greater causing more air to get sucked in.

FAA A&P/IA
LFPN
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