straight 6 - going back to basics

[JZA70 R]

Bit of an embarrassing question here, but i figured some other people may benefit from the answers i hopefully get.

How does a straight 6 engine operate? ...is the image below an accurate representation of piston movement and combustion process? (That is basically what i want to know)



If that illustration is correct then i assume the following

3 pairs of cylinders move in unison (simultaneously) however each pair despite moving in unison, are fired at different strokes?

Does that mean no two cylinders ever fire at the same time (making ignition and injection a sequential process?)

Is this why batch firing ignition and injectors is inferior to sequential because of the wasted fuel and spark?

Does the image above show correctly which cylinders are moving in unison?
eg
1 and 6
2 and 5
3 and 4 ?

Does this mean you could swap two injector plugs (off a batched pair) over to the other and still have the engine run perfectly fine?
(This question pretty much is the reason for all the other questions as im doing some wiring on my 1JZ and noticed the injection loom looks like a batch fire setup... and then that got me to wondering how it all works hehe.)

Thanks

[Norbie]

I believe the answer is "yes" to all of the above.

[JZA70 R]

ok cool, the swapping batch pair injector plugs still has be concerned because although the signal is common among the pair, they have their own individual earths (as far as i can see)

that makes me wonder if they are still being fired sequentially via a switched/controlled earth?

...meh

[JZA70 R]

also is it true 1JZ's batch fire the injectors and 2JZ's are sequential?

would there be big fuel & economy savings switching from batch to sequential injection? ...who wants to work out how much extra fuel is being used on a batch fire injection system compared to the same car using sequential.... as a percentage?

50% extra fuel comes to mind but that cant possibly be right. (if each injector is firing twice as much as it needs to (twice per cycle instead of once) that means 50% to me?
hmmm

[big_zop]

While i cant see the images, i would assume you referring to pairs 1 & 6 (for example) would be on the same stage of the 4 stroke cycle:
Intake > Compression > Ignition > Exhaust for those that dont know.

Thus, if they are both are on the same point in the cycle, then both cylinders would require fuel at the exact same time and if fired from a batch signal as you say, would not actually waste any fuel. Though i do suspect that even though the paired cylinders may match in movement, they might be 360deg out of phase (Intake for one and Ignition for the other) - which would support your claim that batch fire would waste fuel. i would have no idea how much but obviously not enough to justify the use of sequential back then, but maybe today with tighter emission laws.

I have no idea about sequential/batch for 2JZ/1JZ - none of the TSRMs seem to mention it. But this is making my decision/arguement to go 2JZ over 1JZ much stronger...

[sheepers]

i am not sure of this but does batch injection fire 2 shots of fuel into the inlet tract above the valve for each cycle of the engine, each shot being 50% of the requirement for one complete combustion cycle?
if you were firing two lots of fuel (each 100% of requirement) per cycle then you would be running hugely rich?

sheepers.

[thechuckster]

also is it true 1JZ's batch fire the injectors and 2JZ's are sequential?

would there be big fuel & economy savings switching from batch to sequential injection? ...who wants to work out how much extra fuel is being used on a batch fire injection system compared to the same car using sequential.... as a percentage?

50% extra fuel comes to mind but that cant possibly be right. (if each injector is firing twice as much as it needs to (twice per cycle instead of once) that means 50% to me?
hmmm

am not sure - i think that difference is to do with spark control not fuel? A JZ expert may wish to comment.

no - batch firing simply spreads the fuel requirement for each cylinder into several shots that occur during an engine cycle. I will try to find the PDF on autoshop 101 that shows the relationship of injector and spark timing thru the engine cycle (which is 720 degrees or rotation).


edit: spark & fuel delivery timing - see 15th page of this PDF: http://www.autoshop101.com/forms/h22.pdf or 6th page of this PDF http://www.autoshop101.com/forms/h25.pdf .



no - the benefit of sequential injection is injector phasing (changing the injection start time to suit engine RPM and manifold air-speed). This benefit is manifested mostly at idle for cars with large injectors, and at cruise (slightly better economy).

[LeeRoy]

Though i do suspect that even though the paired cylinders may match in movement, they might be 360deg out of phase (Intake for one and Ignition for the other) - which would support your claim that batch fire would waste fuel.

They are 360* apart in the combustion cycle, one is drawing fuel/air on the way down, the other is on the burn stroke.

As mentioned, fuel is divided into 2 parts with one being delivered behind closed valves, and the other being delivered with valves open. Both shots of fuel get burnt, and since theres no place for the fuel to go other than into the engine your not really using any less or more. Although think of the split second of atomisation that might be lost be injecting the fuel 1 revolution earlier than needed.

With Toyota ecu's historically running quite rich a greater gain would be in removing some fuel ie aftermarket ecu, not by removing batch injection.

Some ignition systems also use wasted spark which is on a similar principle to above.

[big_zop]

They are 360* apart in the combustion cycle, one is drawing fuel/air on the way down, the other is on the burn stroke.

As mentioned, fuel is divided into 2 parts with one being delivered behind closed valves, and the other being delivered with valves open. Both shots of fuel get burnt, and since theres no place for the fuel to go other than into the engine your not really using any less or more. Although think of the split second of atomisation that might be lost be injecting the fuel 1 revolution earlier than needed.

Started to realise that they werent the same after Charles had put up the autoshop documents. (and that i too had read them previously... )

It does make sense that although the fuel is injected when it isnt needed, there is no where for it to go unless there is some serious vacuum created by another cylinder nearby to draw it into there. While they may be grouped to prevent this, i am sure i am thinking too much about it and its much simpler than i make it out to be...

[thechuckster]

actually, batched and split injection durations is good for cars that have crap manifold designs (e.g. my 18RTE) ... as (more seriously now) it aids in fuel atomisation via a 'wall wetting' process and the pulse resonance in the intake runners.

Both of those (wetting and resonance) allow fuel to evaporate before entering the combustion chamber. Evaporation also produces a minor charge cooling or increase in air density making for more fuel and oxygen in the combustion chamber.

Wall wetting comes from the air-borne droplets landing on the runner and head intake surfaces. It also reduces incidence of large fuel droplets entering the combustion chamber intact.

The fuel evaporates into the air-stream as it (the air) is both drawn back-and-forward by resonance in the intake runner and the eventual in-rush to the combustion chamber when the valve opens.

With a single injection event, timing is critical and event calculations also have to allow for air-density and air-speed. If too much of the fuel doesn't make it into the combustion chamber you have a lean-even and possibility of engine damage.

[LeeRoy]

With a single injection event, timing is critical and event calculations also have to allow for air-density and air-speed. If too much of the fuel doesn't make it into the combustion chamber you have a lean-even and possibility of engine damage.

This is another key point as it allows headroom for sensors to be slightly off without it affecting the engine dramatically.

I'm taking a guess with this one but couldnt you run smaller injectors also?

[oldcorollas]

what fuel doesn't make it in on one stroke, makes it into the cylinder on the next.. it is not like it disappears it just slightly changes the amount of airvs fuel capour ont he next intake event. but if ECU is tuned for those conditions, and the conditions for that load point don't change.. it is ok.

when you are running at WOT, your injectors might be open 80% of the time anyway, whereas valve might be "open" maybe 40% of the time (288deg cams), or 33% of the time (240deg).. so a lot of fuel at WOT is injected toward closed valve. unless you have injectors that are twice or 3 times as big, to be able to inject in the short time available..

it is not so bad. the fuel helps cool the valve, and the port due to evaporation, and then the air has a cooler port/valve to go past when valve opens.

the benefit of the critical single injection is to be able to run leaner, more reliably.. and that is also the point of direct injection. sequential may not actually be timed to the valve opening events... it might be timed for valve cooling = better evap/atomisation = better economy..

multiple batch kind of "evens things out" so timing/cooling/whatever is less critical..

[JZA70 R]

good info, thanks all.

[SillyCarS]

excellent thread

rep all round

[MWP]

the benefit of the critical single injection is to be able to run leaner, more reliably.. and that is also the point of direct injection. sequential may not actually be timed to the valve opening events... it might be timed for valve cooling = better evap/atomisation = better economy.

This is the main reason why sequential injection was introduced... to reduce emissions.
Squirt the fuel onto the back of the valve when its closed, the hot valve evaporates the fuel = total fuel combustion.

With batch (or other) fuel injection, its possible some small droplets of fuel make it into the chamber and are not completely burnt.