Valve timing and vacuum drop investigation
For some time I've been trying to understand a certain observed nonlinearity in manifold vacuum with respect to Prius engine RPM at highway speed. It seems that in a relatively narrow region centered around 2300 RPM, vacuum drops from 4 - 5 in-Hg down to 2 or so, and otherwise remains steady at one or the other over wide RPM ranges. Since throttle position does not seem to make a large corresponding jump, my interim conclusion had been that the variable valve timing is responsible -- lower vacuum caused by a slight retardation of the intake and thus more of the cylinder charge pushed back out into the plenum. Someone from Prius_Technical_Stuff added the theory that at higher RPM the inertia of air moving through the intake runners would have more of a scavenging effect per intake stroke, thus the valves could be closed later but still provide the same effective compression. But I wasn't completely confident about the cause without getting some hard data. Efforts to read VVTi parameters with a scantool had failed, always returning 0 or some other meaningless, invariant figure. It isn't clear if that's the fault of the ECM misreporting it, or the tool's inability to parse the data correctly. No test or measurement procedures are given in the service manuals, other than simply making sure the sensor waveforms look good. So once again it's time to get down to the real source, and scope the cam and crank sensors that the ECM uses to determine the timing for everything. Both are similar magnetic reluctor devices, issuing a pulse when a piece of ferrous metal passes close by, usually in the form of a tooth on a serrated wheel.
This is a generic [albeit somewhat inaccurate in real life] representation of the cam and crank pulses as we would see them on a scope -- they possess the characteristic shape of rising as the tooth approaches, dropping sharply negative as the tooth passes center, and then returning more slowly back to zero as it leaves. If another tooth immediately follows, the waveform's positive and negative slopes look about the same, but it's clear that the negative slope is the actual finest-resolution trigger point and any positive or zero slope is a "valley" transition from one tooth to the next no matter how long it takes. This is a bit clearer in the actual scope traces below. The 1NZ-FXE mill in the Prius uses a "36 minus 2" crank wheel, i.e. 34 teeth with a gap where two more teeth are missing. This conveniently maps to 10 degrees per tooth slot. The ECM syncs up with the base timing rate of passing teeth and then looks for the missing pulses as an absolute reference as to where the crank is. The cam "wheel" only has three teeth on it [at 90 degrees, thus a "4 minus 1" configuration] and works similarly. Here are some pictures of the real things, courtesy of Galaxee from Priuschat and her husband who spends his days armpits-deep in this stuff and brings home all the coolest discoveries. The ECM uses the relative timing of both as feedback to determine how to control the variable valve-timing actuator, among other things. [Of course everything else derives timing from these signals too, such as when to fire injectors and plugs, and the high number of teeth provides enough resolution to allow misfire detection from small speed variations.]
Exactly when these gaps pass the sensors is apparently not strictly based on #1 top dead center as might be easily assumed; for example, #1 TDC in the Prius engine appears to be when the gap is oriented straight forward at 90 degrees but where the sensor [green arrow] sees it, TDC occurs at the 23rd post-gap crank tooth just as the output signal starts swinging negative. The assumption is made that the positions of these sensor triggers is always the same relative to the crank and cam themselves, so no cumbersome "re-learn" games are necessary as with some other cars. In this case, we don't really need to know the absolute relationship because we're just looking for *relative* intake timing changes while running. There are several basic ranges of valve timing, as shown on this chart from Toyota:
What I'm really looking for is whether my observed vacuum drop corresponds to a transition from range 4 to range 5. Given the way the Prius does throttle management, the engine can be considered under "heavy load" almost any time it is producing meaningful power -- this is one of the things that give rise to its high efficiency. From there, it's all about RPM to effect any change in requested power output, and the remaining question then becomes what is considered "low" or "high" RPM. Another early approach was watching the oil-control solenoid output, which isn't useful either -- other than full retardation at idle and warp stealth that sends the duty cycle to a minimum, it always runs about 40% -- timing changes are done by a very temporary duty cycle shift to nudge the oil valve one way or the other, and that's just too transient to see on a scope. So what's left is to try and scope the relationship between the cam and the crank directly, using the cam signal as a trigger. Even with triggering, getting a stable picture is rather fiddly because the overall frequency keeps changing with engine RPM. But eventually the trace could be stretched just right such that the negative-going slope from the first cam pulse would always line up at the left. Two or more trace times usually overlay the shutter time, often doubling [and blurring, argh] the images. Each of the traces below is a link to a larger picture, in case a closer examination is desired.
Vac at 4 - 5
Vac at 2
Cam seems to correspond with 7.5 crank intervals; an advance of 20 degrees
The pictures aren't really clear enough to determine any difference across the vacuum drop, however.
But nothing says I need to stay on the same timebase as at idle, so I clicked it up a
notch and managed to capture an expanded pair. The cam gap and second tooth are overlaid in two trace cycles.
|2000 RPM||2400 RPM|
Now the relationship is clearer, and we can observe that it is also
between the two speeds. Compare these to the idle baseline, too.
So, that whole theory about valve timing retard affecting vacuum, at least at these RPM ranges, is busted! Now what? Well, the only other possibility is the throttle after all, so I set the laptop to recording RPM and the throttle position over a whole range of driving conditions, and just made a scatter graph out of it. [Select for full-size]
We can easily see the granularity with which throttle position is reported, among other things. A higher vacuum scenario would trend points toward the right and down, while lower would move to the upper left. Now that we've got the viewpoint of long-term trends, there appears to be a small but definite upward tick in throttle opening around 2300-2400 RPM, so that seems more likely as the cause for what I see on the vacuum gauge. But I'm still at a bit of a loss as to why it's not more linear in the first place. It may still have something to do with MG1 transitioning from negative to positive rotation, but I still think that's coincidental. In any case, on the interstates it's the difference between 35 and 25 MPG instantaneous, and it is becoming generally agreed that perceptible MPG gains can be realized by trying to stay in that higher-vacuum region as terrain permits -- not exceeding 2200 or 2300 RPM unless really needed. The sweet-spot article has been slightly updated to reflect this, removing any outright statements about valve retard.