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  #1  
Old 08-08-2018, 08:18 PM
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1001001 1001001 is offline
 
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Default Stupid fuel injection question

I think I understand that in an electronic fuel injection system (with direct injection at each cylinder), the fuel charge is determined by the pressure at the fuel "rail" and the duration of the opening pulse of the fuel injector. This should ensure that fuel only enters the cylinder during the appropriate part of the cycle (intake).

In a mechanical fuel injection system, such as the Airflow Performance system (which I have on the engine package for my -10), the fuel spider is pressurized at all times, but at varying pressures as determined by the fuel servo, so that the flow is determined by the differential pressure across the injector and its orifice.

What (if anything) prevents fuel from flowing during the other parts of the cycle for a given cylinder? Just differential pressure? Does this mean that fuel can flow during all cycle stages, essentially wasting fuel, even in a direct injection system?

For instance, if the fuel pressure is 20 psig (34.7 psia at sea level) (just to pull a number out of lean air) and the intake stroke pressure is, let's say, 20 inches of mercury(about 9.8 psia), we can calculate the flow of fuel through a given orifice. During the compression and power strokes, the pressure will rise well above the fuel pressure, and fuel flow through a direct injector should stop (in fact, cylinder gases should back up through the injector briefly). But when the exhaust stroke occurs, and the cylinder is essentially open to atmosphere again, we would expect a pressure of, let's say at sea level, 14.7 psia. So this would be a 20 psi differential again, and we would expect fuel to flow into the exhausting cylinder.

Does this happen? Am I missing something? Does this kind of system really waste fuel like this?

Forgive my ignorance of fuel injection for I am but a grasshopper.
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  #2  
Old 08-08-2018, 09:07 PM
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rv6ejguy rv6ejguy is offline
 
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The intake valve is there in the equation. Fuel sits in the port until the valve opens. Nothing is wasted.

EFI may or may not be timed to valve opening and the same thing applies- no fuel flows into the chamber until the valve opens.

Direct injection implies the injector is in the combustion chamber and this type involves pressure of thousands of psi generally. Fuel flow into the cylinder is independent of valve opening in this case.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
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  #3  
Old 08-08-2018, 09:08 PM
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Aircraft (gasoline) fuel injection systems are "port" injection designs. The fuel is injected into the intake tract.
Direct fuel injection systems used in some of today's cars inject fuel directly into the cylinder in a timed fashion at pressures over 2000 psi.

Cheers, David
KBTF A&P
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  #4  
Old 08-08-2018, 09:12 PM
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Thanks for the clarification of port injection vs. direct injection. It makes sense now!

Does a mechanical fuel injection system inject at a lower rate continously, and an EFI in one big burst during the intake stroke?
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  #5  
Old 08-08-2018, 09:41 PM
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Quote:
Originally Posted by 1001001 View Post
Thanks for the clarification of port injection vs. direct injection. It makes sense now!

Does a mechanical fuel injection system inject at a lower rate continously, and an EFI in one big burst during the intake stroke?
"Constant flow" is precisely correct for Bendix style and Continental type systems; the nozzle squirts a constant stream of fuel (mixed with a small volume of air) into each intake port. The SDS and EFII brands vary pulse width (i.e. nozzle open time) to regulate fuel delivery, but that pulsed flow is also delivered continuously into each port throughout the entire four stroke cycle. Put another way, both kinds of injection flow fuel all the time, including when the intake valve is closed.
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Last edited by DanH : 08-08-2018 at 09:46 PM.
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Old 08-09-2018, 05:20 AM
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Quote:
Originally Posted by DanH View Post
"Constant flow" is precisely correct for Bendix style and Continental type systems; the nozzle squirts a constant stream of fuel (mixed with a small volume of air) into each intake port. The SDS and EFII brands vary pulse width (i.e. nozzle open time) to regulate fuel delivery, but that pulsed flow is also delivered continuously into each port throughout the entire four stroke cycle. Put another way, both kinds of injection flow fuel all the time, including when the intake valve is closed.
I might mention that port EFI systems termed "sequential" ( a misnomer IMO) may start to inject on a closed valve or open valve as the designers see fit to meet emissions, cold start or throttle response targets.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm


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  #7  
Old 08-09-2018, 05:56 AM
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Dave,

Don Rivera hosts a fuel injection class at the AirFlow Performance shop twice a year, usually in November and March. I highly recommend the class. Not only does Don present the academic side of fuel injection, you spend some time in his labs with pertinent demonstrations.

It's also a good time to get your injectors balanced while you are there.

bob
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  #8  
Old 08-09-2018, 06:12 AM
rv7charlie rv7charlie is offline
 
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The common terms for what Dan & Ross are describing are 'sequential', for electronic systems that time each injector's firing relative to its respective valve timing, and 'batch fire', for electronic systems that fire all (or a bank) of injectors at the same time, regardless of respective valve timing. (Not saying those are the best terms; just the common parlance.) AFAIK, all the 'off the shelf' electronic systems being marketed for homebuilt a/c are 'batch fire'.

Systems like the Megasquirt can be set up to do either method, but you'd be on your own setting one up. It has been done by individuals, though. Dan and a few others are running Megajolt systems, the ignition-only variation.
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Old 08-09-2018, 06:58 AM
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Quote:
Originally Posted by rv6ejguy View Post
The intake valve is there in the equation. Fuel sits in the port until the valve opens. Nothing is wasted.
Well, 'wasted' in this case means a stoichometric mixture richer than what is required for the engine to run at the operators requirements. So yes, as we all know, mechanical systems, like carb systems, provide fuel at a "onesize fits all" ratio, with an open loop system design. EFI, allows that flow to be tuned better reducing over rich ie wasted fuel conditions mostly because of its a closed loop system. Which is why we get better fuel economy from EFI...... at the expense of introducing an electrical "component" into the system and its associated complexities......

But yes, wrt aviation systems, they all are flowing fuel constantly......just better mouse traps.
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  #10  
Old 08-09-2018, 07:04 AM
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Quote:
Originally Posted by maniago View Post
Well, 'wasted' in this case means a stoichometric mixture richer than what is required for the engine to run at the operators requirements. So yes, as we all know, mechanical systems, like carb systems, provide fuel at a "onesize fits all" ratio, with an open loop system design. EFI, allows that flow to be tuned better reducing over rich ie wasted fuel conditions mostly because of its a closed loop system. Which is why we get better fuel economy from EFI...... at the expense of introducing an electrical "component" into the system and its associated complexities......
While all automotive EFI systems have used closed loop AFR control for about 30 years now, typically we are not using closed loop in aviation with leaded avgas. With a switch to unleaded, that's all possible. We already flew an RV10 back in 2007 with this technology.

The Bendix type injection works pretty well in cruise and high power with balanced nozzles actually. We see the majority of fuel savings using EFI in the start/ warmup/ taxi/ descent phases or at low power settings up high where the mechanical injection does not meter very accurately by comparison.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW since 2003- 424.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm


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