Jazsun

I have a 93 so it uses Batch Fire instead of hte sequential port fire like all the 93+ LT1's what the differences, and advantages of both? I heard that the only advantage to be had w/ sequential is better economy.

Jazsun

Batch Fire - Batch fire means that 2 or more injectors are triggered at the same time once every crankshaft revolution. If the ECU is synched with the engine's cycle, the injection timing can only be half optimized as fuel is injected both on the intake stroke and on the power stroke. Companion cylinders are paired in batch fire mode similar to wasted spark ignition modes. The advantage of batch firing is that the ECU needs only to know where TDC is. This means that a sync on the cam is not required. The disadvantage to batch firing is that the Injector Dead Time is doubled for the engine's cycle. This leads to a decrease in fuel flow and typically requires a larger, less efficient injector to be used to make up for the loss of flow. On High Horsepower applications this means the idle quality will suffer tremendously.

Injuneer

In the 93 LT1's the ECM fires all 4 cylinders in each bank simultaneously, once per crank revolution.

The "economy" advantage of sequential injection (each injector fires one per two crank revolutions, timed on an "end of event" basis to the intake valve closing) is going to show up mainly at low/part load operating conditions. That's when the injector duty cycle is low enough that it can fire mainly on the intake stroke. Once you go to max power/rpm at WOT, the injectors will be open most of the time, during all 4 strokes, whether the injection is sequential or batch fire.

I've actually had a chance to run my sequential 94 with the injectors timed one crank revolution out of phase (thanks to the Electomotive Opti-Eliminator/SDI setup), and other than a light stumble when the engine was very cold, it was impossible to detect any negative affect, at least not SOTP.

arnie

Because the duty cycle will increase with RPM, even without an attending increase in fuel requirement, sequential can become batch, as low as ~ 3000 RPM, depending on how well matched the injectors are to engine. IOW, the WOT designation can be as low as ~ 3000 RPM, so as 'Injuneer' noted, under full power, (other than very low RPM) there is virtually no sequential injector firing.

arnie

I'm not sure I understand what you stated here. Could you explain this wording in more detail?

Injuneer

I think what he is trying to point out is that with a batch fire system, each injector fires twice as often as it would in a sequential system. Since the injector cycle time includes a productive portion and a non-productive (or dead = open/close) portion, when you fire the injector twice per two crank revs, you have 2X the dead time. That forces you to increase the injector size, to reduce the duty cycle.

Now you turn the injector down to a small enough pulse width to achieve idle, and you have two problems. You are trying to set a pulse width that is roughly 1/2 of the pulse width used for a sequential injector, and you have a large injector which also requires a shorter pulse width at idle. End result is difficulty in achieving the minimal flow required for idle.

I'm not sure why the larger injector is referred to as "less efficient". The rest makes sense.

arnie

OK, I follow, and agree, at least at low load/part throttle anyway.Hmmm. We know sequential becomes batch (as it injects fuel behind a closed valve), and near constant open at WOT conditions. Does effectively becoming batch, in itself, increase the injector size requirement? I don't believe so. Does the OEM '93 indeed have larger injectors, compared to the '94?I agree with this part. Thanks for reply, Fred.

Injuneer

But the sequential system is opening the injector once, and holding it open for most of the 4 strokes (2 crank revs), then closing it once. The batch fire injector is opening, spraying, closing, opening, spraying, closing during those same 4 strokes.

The 93 actually has smaller injectors than the 94, but they are more than large enough to avoid a 100% duty cycle at the stock HP rating.

kevm14

For academic's sake, we could call the 93 LT1 "bank fire," a subset of "batch fire." TPI was also batch fire, but didn't they only have one injector driver? That would mean every injector fires at once. I understand this can have fuel atomization benefits as the fuel will have a little time to "mix" around with the flowing air in the intake before getting sucked into whichever cylinder.

arnie

That extra time you are referring to, is time with the valve closed. How well is that air 'flowing'?

Injuneer

I don't think there is an issue of "atomization".... the fuel is going to flash to vapor phase when it hits the back of the hot intake valve. I'd be more concerned about the fuel vaporizing, then "drifting" out of the port as it expands.

When the guys who built my engine explained a few things to me, they pointed out the benefits of sequential injection, with very large injectors, and the ability to flow fuel preferentially when the intake valve is open. The MoTeC ECU has a table for injector timing, based on "end of event" back calculation of the correct time to start the pulse. They claimed they could alter power by altering the injector timing relative to intake valve events. They also used the same approach with their strong preference for dry nitrous. With a dry system, mixing the air/nitrous in front of the plenum, nitrous only flows into the intake runner when the valve is open. With a wet/direct port system, you're flowing cold nitrous and fuel against the valve continuously, in addition to any fuel that the injectors spray.

Maybe its more theory than practicality. I don't think most of us could tell the difference between a batch fire and a sequential system, at least not SOTP.