Priorities: RPMs vs VAC

[PaleMelanesian]

I had something to say about rpm and vacuum, but that hardly seems relevant here anymore.

[user removed]

Most efficient is low RPM and low vacuum.

1200-2400 with vacuum of 2 inches is pretty close on most engines for acceleration.

Throttle below 80%.

regards
Mech

[jfitzpat]

Originally Posted by Superturnier

How much does the amount of charge in the cylinder affect to temperature?

Is the amount of mixture more significant than the A/F ratio of the mixture?

Temperature of what? If you are measuring the temperature of exhaust, the amount of mix is significantly less important than composition. This is why the 'constant EGT climb' method works in small, normally aspirated aircraft.

If you are looking at CHT's and work, it depends. At it's simplest, CHT's depend more on the point of peak pressure than anything else, so the primary impact of the change in mix is the change in the behavior of the flame front and the associated change in the pressure curve.

Regarding lean burn:

I have a coworker who is very experienced in this area. It is very interesting, but the graph is a little deceptive. Picture the NOx line shifted just a tad right. Stoich is more on the rising slope than near peak. So, the drop is not nec. as substantial as it seems on the graph in the article. Also, emissions remains a problem. You have to setup the catalyst reaction to turn the HC's to water and CO2, and it is easily contaminated. That is why things like 'low sulfur' diesel are so important.

Also, in that range particulates soar. The cat cannot clean that type of carbon, so you have the headaches of a DPF setup (I'm actually peripherally involved in the development of a new particulate sensor method now).

Regarding off topic:

Sorry, this stuff should have been split.

Regarding on topic:

I would, with a few caveats, agree with Old Mechanic for a conventional gasoline engine.

-jjf

[PaleMelanesian]

Originally Posted by Old Mechanic

Most efficient is low RPM and low vacuum.

1200-2400 with vacuum of 2 inches is pretty close on most engines for acceleration.

Throttle below 80%.

Pretty much. I'd trim the top end of the rpm range, to maybe 1200-2000 instead.

Throttle can be anything below where the ecu goes into open loop and enriches the mix. My standard target is 80% throttle, but always below the 95% open loop cutoff on my car.

In the original context - if you're having to choose between low vacuum (high MAP) and low rpm, you need to slow down. Then you can optimize both at the same time.

[user removed]

In my VX lean burn stops at 2500 RPM, so I try to use lean burn almost exclusively. I can feel the power surge when I go beyond the lean burn threshold with throttle position, so in this case it is well below 80%, but that is just becasue of the lean burn feature of the VX. The VX pulls well enough from 1000 RPM in 5th gear at 30 MPH.

In Pales instance, lean burn is not an issue, and the proof of his point lies in his mileage.

Larger displacement long stroke engines should use lower RPM.

The other day I was accelerating next to a fairly large diesel box truck. We were shifting gears and accelerating at almost exactly the same pace until I moved ahead in 3rd gear, which in the VX will red line at close to 120 MPH.

regards
Mech

[jfitzpat]

Originally Posted by Old Mechanic

In my VX lean burn stops at 2500 RPM, so I try to use lean burn almost exclusively. I can feel the power surge when I go beyond the lean burn threshold with throttle position, so in this case it is well below 80%, but that is just becasue of the lean burn feature of the VX. The VX pulls well enough from 1000 RPM in 5th gear at 30 MPH.

Is your Honda using the older CVCC system? I mean, does it use the parallel fuel path? I've read about them, but never driven one (that I can recall). But I've looked at the newer Honda lean burn system quite a lot and the transition is very subtle. I'm not sure I would be able to to detect it by feel.

-jjf

[user removed]

I owned a 77 Accord that had the CVCC (compound vortex controlled combustion) engine.

It used two intake passages, with 2 intake valves and 1 exhaust.

The smaller passageway was a completely separate induction circuit from the main passage. The carburetor had a dedicated circuit for the small passageway and there was a prechamber in the cylinder head, somewhat similar to a diesel.

The small circuit prechamber ran at an AF mix of 12 to 1, while the main circuit ran at 18to 1. The smaller circuit was initially ignited by the spark plug, then the flame front transitioned to the leaner main circuit.

Not very powerful engines but they did not require catalytic converters to pass then current emissions regulations.

The VX is very different. It Has 4 valves per cylinder and VTEC-E, with port fuel injection. Lean burn is accomplished by only slightly opening one of the intake vales under 2500 RPM, which produces a high swirl action and better distribution of the fuel and air mix. Combined with a special Oxygen sensor (very expensive) it can run at mixtures as high as 22-24 to 1 at low revs and low throttle positions.

I could not feel any transition in the 77 Accord because there wasn't really a transition to feel. In the VX it is obvious, when the transition occurs. Its almost like an old 4 barrel when the secondaries opened up.

I also owned an 84 CRX 1.5 that I bought new. Serial # 1018 made in June of 83. I think it was also a CVCC design. Without even trying to get good mileage I averaged 44 MPG for the 50k miles I drove that car.

As emissions tightened the CVCC was replaced with fuel injection a couple of years after that 84 model.

regards
Mech

[gone-ot]

...my 1984 Civic DX had CVCC and consistently got 42-45 mpg (@ 55 mph with A/C) before "eco-modding" became popular.

[nj1266]

Hi jjf,

He works for Innovate...there I said it and not him. I have owrked with him in the past when he designed a plug-in for my XEDE piggyback on my Evo. Listen to him, he knows what he is talking about. I have learned a lot from him and Klaus (the genius behind innovate).

I use innovate widebands in all my tuning applications. I tune Evos and Subarus using opensource tools. The Innovate wideband is the only one that I use. I have two of them; one on my tuning kit and one installed permanently in my Evo. The latter helps save fuel when I pulse and DFCO. As you know, in DFCO mode the injectors shut down and the engine becomes an air pump.

Great discussion btw. As far as saving fuel, the best cruise AFR is around 15.2-15.4:1. I have been able to get my ECU on my Evo to run in open loop all the time at 15.4:1 AFR in cruise by changing the closed loop tables. It is a tuning trick that I have learned. I have managed to get 27 mpg in my 330 hp Evo w/o pulse and DFCO by advancing the timing at cruise and running leaner AFR than stoich. I tested the old fashioned way by filling the tank driving till it is empty and re-filing and calculating the mpg. This was in mixed 30/70 street freeway driving.

I will test with pulse and glide soon and see what I can get.

[jfitzpat]

Sorry, I thought I had replied to this, but I don't see it:

Originally Posted by Old Mechanic

which produces a high swirl action and better distribution of the fuel and air mix.

FWIW, the high swirl is more related to how it effects the flame front. There are always little vortex from filling the chamber, but they subside fairly quickly, so at low load/rpm, most engines have a fairly laminar flame front. When you are running very lean mixtures, this becomes a problem of time, since they reach the 50% burn point very slowly.

What a vortex does is it actually spirals the flame front, giving it greater surface area. They was some incredible images of this in a trade journal just a few years ago, but I can't find it at the moment.

We've known this helps at higher RPM's for a long time, since it flattens out that part of the P/V 'box', but that design brings the benefits down to low RPM/load situations so that they can be run much leaner.

Originally Posted by Old Mechanic

Combined with a special Oxygen sensor (very expensive) it can run at mixtures as high as 22-24 to 1 at low revs and low throttle positions.

They call this their "Linear" O2 sensor. It is actually a fairly impressive feat of materials engineering, but inherently expensive. It's interesting to us, because of our 'Digital Direct' measurement method. We end up solving the same problem, but by accident.

Typically, a O2 measurement is made by pumping or withdrawing O2 to stoich. The current is then reported as a measurement, and is inherently non-linear. Klaus was frustrated because of the speed. It is akin to a PID motor controller. You are always really looking at a moving average and are only accurate under constant values that saturate the PID loop. In an engine, AFR is really always changing.

In looking at the Nernst equation, Klaus realized that the sensors should follow a certain behavior if, instead of driving to stoich, measurements are pulsed between two reference points. Right away, it was clear that he got the speed he was looking for (you can see valve action or even individual misfires), but it also turns out that the response doesn't have to be fitted to a curve, it is effectively linear. This matches the original math, but surprised him (and me, when I first tested it for myself) nonetheless.

The whole objective behind Emisense is to allow techniques like Honda's, or various lean burn's etc, to be executed with cheaper, more robust, sensors.

-jjf