VWjet

In reading the hardcover book Mazda sent out, I have a question regarding this topic in the book:
So, if you maintain engine speed below 3750 rpm, will your fuel consumption be reduced dramatically or is the engine computer going to take over and compensate when you upshift at 3750?

Also, why would the second two injectors be only 4-hole type instead of 12-hole like the first one? Cost, I would imagine.

Y&Y

I don't think the number of holes has to do with cost entirely(or at all)

If you re-read the quote you have posted you will see that that the first injector has 12 holes for finer fuel atomization. Meaning able to inject fuel with better precision to save gas. And because the trichoid rotors are already in motion it would take less fuel to continue its momentum and to increase its rpm it would need a bit more fuel. As for the Primary injector No. 2 and its secondary injectors, would have four holes but not as small as the 12 hole, would probably not need to have the fuel to be injected as finely as the Primary no. 1 injector to keep the rotor moving.

Imagine you're a caveman trying to push your oversized wheel made out of stone. At first it would be hard to push and you would be using a lot of energy to get it moving. But once it starts to move you would need to use less energy to keep it going at a steady pace. And if you want it to go faster you would steadily add more energy into it. Now if you can control exactly how many calories to burn to move the stone wheel initially you would not tire out as quickly.

But this is all speculations.

Damn it wheres the mechanical engineers to explain this?

j9fd3s

with the older rotaries the trick to good fuel economy was to keep the engine in the rpm and load zone where the ecu would run in closed loop. that is also co-incedent with the secondary injectors not being on. granted on the older rotaries they run a very slow computer and they have some band-aid tuning to get around it. the rx-8 probably wont be saddled like that, and theres no turbo either so it might act like a "normal" engine where the more rpms the more fuel. the mazda rotaries are just about the only cars that have staged injection.

mike

TerenceT

my GUESS is it has to do with duty cycles of the injectors

at 3751-9000 rpm, the duty cycle of the injectors might be 65%-105%

it became a problem of choking if the fuel atomize but not enough fuel in the chamber

fine atomized fuel makes it burn better (at low speed) for better emission.

MikeW

At 3750 and less rpm, there is only one intake port online.
So you should hopefully get 25mpg @ 75 with only one port/rotor working.

Most people only operate their engines in two kinds of states. 1. low speed/low load 2. High speed/high load (this kind of explains the VTEC)
The 4 hole injectors can move more fuel than the 12 hole, which is okay because at low speed the engine can not use that much fuel (even if you floor it in 6th gear)

Farsyde

i think mazda just decided that instead of having one massive injector that would prolly bleed excess fuel and kill gas mileage they installed stages to very accurately control the fuel delivery

j9fd3s

well they have done it. the 83-85 efi rotaries (12a turbo and gsl-se engine), have 2 big injectors, and approximately 140hp.
as time went on they wanted more power, but you need larger injectors, and larger injectors have a harder time delivering the small amount of fuel to idle, so for 86 they went to a staged injection setup, that lets them have smaller injectors for idle and low speed, and bigger ones for high rpm.

mike

kidmarc

Not easy to answer since you have little data to begin with. You need to know the flow rates of the injectors and the difference/performance of the ultra-fine injectors to the others.

Generally, the one set of injectors will be more fuel efficient than the 3 sets combined. However, dramatically would depend on your definition of the word. Generally costs are the reasons. The flow rates are not known to you, and flow rate is not the only variable so at this point cost is the reason.

Peace
marcus

rotarygod

Its all about fuel economy and emissions. A smaller fuel injector will have a finer mist like spray pattern than a larger injector even if is tuned to flow the same amount at the same rpm. The finer mist is easier to atomize into the air stream. This leads to a more efficient use of the available fuel and less waste. Less waste equals less fuel needed. This is also the reason for 12 holes as opposed to 4 at the higher rpms. As rpms rise the importance for a finer spray pattern start to diminish. There is more velocity in the air and it has more energy to carry and disperse the larger droplets of fuel. At the same time, it is also beneficial to move the injectors farther away as rpms rise. velocity is not very high at low rpms so the injector needs to be right at the engine to give the fuel as little time as possible to fall out of suspension. As the rpms rise the air intake velocity is such that the fuel can be injected into the stream much farther away and stay evenly dispersed in suspension without falling out of it. This also results in more even mixture of air to fuel.

kidmarc

A fine(r) mist is already atomized. The next step is vaporization.

The importance for a finer spray never diminishes unless you are at full vapor--methane, or your definition of importance is purely subjective.

Peace
marcus

Farsyde

Atomizing the fuel is important for any time in the engine; low or high rpm. However, the turbulence of the air coming into the engine at say 7000 rpm's would be enough to separate larger drops of fuel into smaller ones. Therefore, atomization is "less" important. It's all words man i guess i can see where it's alittle confusing though, but not incorrect.

kidmarc

Well, I was merely trying to clarify when it comes to high mileage; not stipulating that he is incorrect. That is why I stated one's definition of 'importance' would be subjective.

From 'standard fuel economy' point-of-view, Rotarygod is correct. From high mileage point-of-view, refining the spray does not lose importance until one crosses over to dry vapors... and finally TCC (Thermal Catalytic Cracking).

Peace
marcus

rotarygod

OK you over analyzed what I meant to put into words but yes thats what I am talking about.