Fuel Injections

Question 1

What are the principals of operation of fuel injection systems?

Answer 1

Nozzels discharge atomized fuel into the air stream entering the intake manifold (or in the case of diesel engines, directly into the intake of each cylinder)

The throttle is also connected to the fuel metering valvue. This automatically calibrates the fuel/air mixture

Question 2

How does a fuel injection system know how much fuel to send to the cylinders?

Answer 2

The Fuel Control Unit (FCU) senses where the throttle valve is at and so knows how much air is coming in and sends this info to the Fuel Distributor (with the fuel pump) to dispense the correct amount of fuel into the intake manifolds of each cylinder.

Question 3

Why is a fuel injected system better at atomizing fuel and what effect does it have on the engine compared to a cabureted system?

Answer 3

Because the fuel pump sends it to the fuel distribution under pressure it gets more thoroughly atomized. This means we get a more uniform mixture across all cylinders, which gives more even power distribution as well as more even temps across cylinders

Question 4

What are the advamtages of a fuel injected system vs a carbureted one?

Answer 4

• More uniform fuel distribution to all cylinders
• More power, since the need to heat the carbureted air is eliminated (when you pull on carb heat you lose power because you enrich the mixture)
• Better cooling across all cylinders
• Less susceptible to icing (only impact and throttle icing)
• Save on fuel because of more regulated flow of fuel
• Faster, more accurate throttle response due to fuel being directly injected into te cylinder (Because the fuel sensor is well ahead of the nozzels)
• Easier to start in cold weather (AVGAS doesnt atomize well even in warm weather, this is why a carb plane needs to be primed, to spray a bit extra fuel)

Question 5

What are the disadvantages to fuel injected systems?

Answer 5

• Starting a hot engine can be more difficult
• During ground ops on a hot day, vapour lock may occur
• Engine restart may be difficult if the engine quits due to fuel starvation (same situation as with vapour lock)

Question 6

Why can it be harder to start a hot engine on a fuel injected system?

Answer 6

Because of vapour lock.

When you shut down an engine the moving fuel that was keeping the fuel lines cool heats up with the massive heat rising off the engine. This can cause the fuel to start evaporating, leaving pockets of air in the lines.

So the engine may start initially if these is a bit of fuel left in the line, but then nothing but air gets injected into the cylinder and the engine dies again. If theres lots of pockets this can happen a few times in a row.

Question 7

How can vapour lock be avoided?

Answer 7

• Once you shut down the engine, open the oil door to let the heat escape
• Park into the wind to help cool the engine

Question 8

What is in place to deal with impact icing over the air intake points?

Answer 8

Alternate Air inside the engine cowling.