We’ve had some questions this week about injection strategies and mountings used in aircraft EFI systems since there seems to be some confusion about these topics.
At SDS, we’ve always been focused on performance and reliability. For the last 25 years, we’ve used the non-sequential strategy on all our aviation and automotive systems because it’s simple and reliable and provides the same power potential as sequential. We’ll continue to use the same injection strategy on the new EM-6 currently under development.
What’s the difference?
Non-sequential strategies vary, injectors may be fired in ones, twos, threes or most commonly, all at once which is usually called batch fired. On our 4 cylinder, aviation systems with individual cylinder trim, injectors fire one at a time, in sequence, so there is some confusion as to what to call this strategy but SDS aviation systems are technically not batch fired as some have erroneously stated. Injectors are not timed to valve opening however and don’t require a cam sensor.
Sequential injection is an unfortunate misnomer (should be called timed IMO) but generally means that injectors are triggered in relation to valve opening on that port. This strategy was developed in the 1990s by auto OEMs to aid emissions and part throttle driveability, especially when cold. Sequential injection requires a cam sensor and one drive transistor per injector.
Semi-sequential or bank fire is another strategy which is more like batch firing because no cam sensor is fitted, so injector firing cannot be timed to occur while the valve is open and port air flow is established. Part of the fuel may be injected on a closed valve and part on an open valve.
In dyno testing at high throttle and rpm, there is no difference in power between any of the 3 basic strategies. We had one of our race engine builder clients in Okinawa put the same Nissan SR20DET engine on his engine dyno with SDS and Motec EMSs on the same day since he was a dealer/ tuner for both brands. The result was that the engine pulled the same power, ± 2hp in each instance on this 500hp engine. Other people have found the same result in their dyno testing.
Sequential is superior for low rpm/ low load emissions- which is what it was developed for. Since aircraft engines don’t spend much time at low rpm and MAP and emissions are not a concern. We don’t consider that sequential is of benefit here, given the extra complexity.
Sequential, in most instances, becomes not timed to the valve open period at high MAP/ RPM anyway because there is not enough time to get all the fuel into the port during that time span. Under these conditions, the system must start spraying fuel on a closed valve. This is the main reason why there are no power gains at high rpm over batch or semi sequential firing.
To realize the full potential of sequential injection, you'd have to dyno the engine to determine when to start and stop the injector open period. You can’t really guess at this. This adds another layer of complexity.
For best power, all that has to happen is that the air/ fuel ratio is optimal and the spark timing is in the right place for peak cylinder pressure to occur at the most advantageous crankpin angle.
Interestingly, one of our Reno race clients was told by the Motec dealer (he was considering both SDS and Motec on advice of his engine builder) that you couldn’t trim the fuel amount to each cylinder individually without a cam sensor. That’s interesting, but not true with SDS, since we’ve been doing it since 2016… All you need is one drive transistor per injector. He went with SDS after seeing Andy Findlay win Sport class last year with the EM-5 system. Motec could be forgiven for this erroneous advice since they don't know SDS any more than I know their system.
In the end, the intake valve lets the fuel/air mixture into the cylinder to be ignited at the right time and injection strategies don’t matter much on aircraft applications.
At SDS, we’ve always been focused on performance and reliability. For the last 25 years, we’ve used the non-sequential strategy on all our aviation and automotive systems because it’s simple and reliable and provides the same power potential as sequential. We’ll continue to use the same injection strategy on the new EM-6 currently under development.
What’s the difference?
Non-sequential strategies vary, injectors may be fired in ones, twos, threes or most commonly, all at once which is usually called batch fired. On our 4 cylinder, aviation systems with individual cylinder trim, injectors fire one at a time, in sequence, so there is some confusion as to what to call this strategy but SDS aviation systems are technically not batch fired as some have erroneously stated. Injectors are not timed to valve opening however and don’t require a cam sensor.
Sequential injection is an unfortunate misnomer (should be called timed IMO) but generally means that injectors are triggered in relation to valve opening on that port. This strategy was developed in the 1990s by auto OEMs to aid emissions and part throttle driveability, especially when cold. Sequential injection requires a cam sensor and one drive transistor per injector.
Semi-sequential or bank fire is another strategy which is more like batch firing because no cam sensor is fitted, so injector firing cannot be timed to occur while the valve is open and port air flow is established. Part of the fuel may be injected on a closed valve and part on an open valve.
In dyno testing at high throttle and rpm, there is no difference in power between any of the 3 basic strategies. We had one of our race engine builder clients in Okinawa put the same Nissan SR20DET engine on his engine dyno with SDS and Motec EMSs on the same day since he was a dealer/ tuner for both brands. The result was that the engine pulled the same power, ± 2hp in each instance on this 500hp engine. Other people have found the same result in their dyno testing.
Sequential is superior for low rpm/ low load emissions- which is what it was developed for. Since aircraft engines don’t spend much time at low rpm and MAP and emissions are not a concern. We don’t consider that sequential is of benefit here, given the extra complexity.
Sequential, in most instances, becomes not timed to the valve open period at high MAP/ RPM anyway because there is not enough time to get all the fuel into the port during that time span. Under these conditions, the system must start spraying fuel on a closed valve. This is the main reason why there are no power gains at high rpm over batch or semi sequential firing.
To realize the full potential of sequential injection, you'd have to dyno the engine to determine when to start and stop the injector open period. You can’t really guess at this. This adds another layer of complexity.
For best power, all that has to happen is that the air/ fuel ratio is optimal and the spark timing is in the right place for peak cylinder pressure to occur at the most advantageous crankpin angle.
Interestingly, one of our Reno race clients was told by the Motec dealer (he was considering both SDS and Motec on advice of his engine builder) that you couldn’t trim the fuel amount to each cylinder individually without a cam sensor. That’s interesting, but not true with SDS, since we’ve been doing it since 2016… All you need is one drive transistor per injector. He went with SDS after seeing Andy Findlay win Sport class last year with the EM-5 system. Motec could be forgiven for this erroneous advice since they don't know SDS any more than I know their system.
In the end, the intake valve lets the fuel/air mixture into the cylinder to be ignited at the right time and injection strategies don’t matter much on aircraft applications.
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