Does someone have an idea how can be easily estimated the VE of an engine be it rotary or piston for any given speed?
As this is directly connected to the maximal resulting compression pressure, knowing the VE would provide a tool to theoretically determine what boost pressure at given speed is save, all other fluid variables preserved.

 

There's two basic problems.

One is that, while you can compute VE experimentally given a dyno plot and numbers for brake specific fuel consumption at various RPMs (which *is* measureable) doing it theoreticially is, to put it briefly, a non-trivial task.

Second is that if you come up with a theoretical maximum boost pressure at any given RPM (which isn't simple either really, but comparatively, sure) there's not really much way to do anything useful with it... it would be nice to vary boost to run combustion chamber pressures at the bleeding edge everywhere on the powerband, but even with stuff like fully electronic wastegates, response time isn't fast enough.

 

VE. is a function of "State of Tune".
It varies with Rpm.
Increasing until peaking at the Rpm. range in which "scavaging" becomes effective, (in resonance) then procceds to fall off.
Depending on state of Tune, peek % will take place at different points in the usable Rpm. range.
Although a major factor in reciricating engines, static compression ratio in NOT a prominent factor in determening VE. in a Rotary engine. Specially in a Turbocharged Rotary engine.
Theoretical or practical, a max. "safe" boost level, has little to do w/ VE. but rather with a variaty of other factors including but not limited to: Comp. ratio, operating Temps., IC efficiency, Charge air temps., Exhaust back presure, Ignition timing, Fuel quality & Octain rating, Intake track's frictional loses, Port timing, Plug heat range, EMS resolution,......... so on & so on ..........
What ever the Rpm. in which best VE. occurs on either a Rotary or piston engine, adding a Turbo will simply magnify that same window of operation.
Obviously, more boost, more power in a wider widow.
Many tuners have achived great success running 20+ psi. boost on pump gas in every day driven cars making 500+ RWHp. with total reliability.
Tuner's proven conbinations, & attention to proper blue printing, are the major players here.
VE. is an important factor on NA. engines tuned for Hi power w/ Fuel economy as a prime consideration.

GT1-20b

 

Detonation is one of the main factors limiting possible boost levels. The point at which combustion mixture detonates is a function of pressure, temperature and fuel detonation properties. The level of compression PRESSURE and temperature reached within the compression cycle is given by the compression chamber geometry (most important characteristic been Compression Ratio) and the mass of gas (mixture) been compressed. This mass could be expressed as function of the mixture temperature and volume. The volume is given by engine’s VE at respective speed.
Now, if we know an engine’s VE (hereby I speaking about VE measured at constant induction pressure –load) for given speed and keep the charge temperature constant then we know how much gas is induced at any speed, and therefore we can asses what compression pressure is been reached. If all other variables stay unchanged, there is a limit pressure at which detonation stars. Theoretically for best power we aim to tune an engine to work at the edge of this limit. However if the induction pressure is the same at all speeds, then this limit is reached only at engine speed where is the maximal VE engine speed (at the point of the engine’s best torque). If we have a form of well-managed force induction though, we can change the induction pressure in compliance with VE in order to induce equal maximal amount of gas (mixture) at all engine speeds. This is the way the engine of Audi A4 1.8T works providing flat torque curve within the 2000 - 5000 m-1 band

 

The Audi S4 Turbo, runs VVT. (variable valve timing), & a 2 position variable intake lenth manifold.
Both of these techs. designed for managing Intake air velosity. When combined, they radically enlarge the window of operarion targeted by the designer.
Torque & VE. are determined by Intake Air velosity, therefore keeping velosity at a fixed speed throughout a wide range of Rpm. will by default maintain VE. % @ a constant throughout that Rpm. band.
2 cam heads w/ VVT can control air velocities & their corresponding harmonics to what ever the designer or tuner have in mind.
These are luxturies not found in Rotary engines.
Although atempts at these techs. have been aplayed to Rotaries, ie: 6 Port Induction, & Dinanic Resonance Chanbers, they yeild results different from that of recipricating engines.
While in the pitron engines, velocities are keept relatively equal through a wide range of Rpm., in the Rotary these technologies yeild multiple increasing VE. curves, BASED on Rpm.

Detonation can occur in as little as 5 psi. boost, It can happen even if NOT in boost, if on the hi side of stoch.
A/F ratio, Fuel quality, and all factors mentioned previously, will affect the point of self ignition by combustion presure & temp. but in the real world, these factors are never keept equal.
In sumary, as pertaining to performance Rotaty engines, a "max. safe boost limit" is only particular to the "State ot Tune" of "A" particalar engine, and the supporting hardware arround it.

 

Theoreticially, yes. In practice, you can't do it. The Audi A4 does vary boost to create a flat torque curve, but it's not running anywhere near the limits of detonation anywhere on the powerband, a fact which is made blatantly clear when people get so much more power at the top end of the curve by using manual boost controller at the same peak boost as the factory settings. You can't accurately control boost as quickly as the engine can change speeds! There've been all sorts of attempts to do so; electronic wastegates, etc. Porsche didn't make them pan out, nor did Audi. You can do something similar but you have to run enough on the safe side that it doesn't matter.

 

Oh, I don’t mean the A4 1.8 T pushing only150 HP runs somewhere near the edge. This engine was also put in Audi TT where the max power is I think 225 HP. The A4 is just an example what can be achieved by varying the induction pressure.
Certainly you can’t change the induction pressure fast enough, but you can have a target pressure for given speed. The point is that knowing the max VE and the speed where it is, consequently the max torque, you can aim to similar torque at any speed by raising the induction pressure or changing other variables limited by the compression pressure (mostly timing advance). It is not uncommon practice to gradually slightly raise boost or/and advance after the speed where max torque occurs. Knowing the VE would just give an idea how much the raise could be, having in mind the desired safety margin

 

The VW motors put a lot of other efforts into having a broad torque curve, so the engine hits close to its max VE fairly early on and falls off. The boost pressure doesn't have to vary that much.

Well, then try something that would, NA, hit peak VE at 6k RPM. This is the kind of thing that would most benefit from jacking boost pressure up at lower RPM, but VE values might vary by a factor of 2... meaning obviously, boost pressure has to change a lot. Well, okay, how fast can a turbo slow down? You can predict that (actually, saw an SAE paper on predicting transient response of turbos; should see if I can get ahold of that) and figure out what boost levels you have to command at various RPMs... keeping ahead of things... but the engine angular acceleration is going to vary in different gears, so you need to look at that too. Plus, fun hypothetical situations like what happens if you are at full boost, lift a tire for a moment, gain RPMs out of proportion to what the computer expects, then regain grip?

'course, you can artificially control VE through fly by wire throttles, but that's not all that great a solution... you can plan on dropping spark timing a lot to prevent detonation when boost goes over your target values, which does wonders for efficiency (especially considering the higher intake air temperatures; no ideal intercoolers in this world) or you can just lower the boost targets enough where it's not a problem.

Overall, I fail to see the point; why not just, say, keep the engine in its powerband with that transmission thingie?


Easy estimation of VE... okay. Simplest method is to look at port flow as a function of vacuum and given rotor position... then you just look at the volume of the working chamber as a function of rotor position, use the change in volume with respect to time to come up with an approximation of the pressure change in a given time, look at port flow at that pressure change and time, and integrate the whole thing over the period that the port's open. Then, obviously, just divide that by the swept volume. Of course, that ignores all *SORTS* of important things... pressure wave tuning, exhaust scavenging effects, etc.

Come to think of it, sticking an engine on a dyno and looking at an airflow meter might be simpler.

 

Not bad, Not bad at ALL !!

 

Heh. Truth to tell, I need to sit down and do the math for all of that m'self... I swear, I'm probably going to be the only engineering student from around here to take his books with him on vacation, for fun.

 

OPINION, it seems that a rotary can consume air like a 350 chevy, or shall I say pass as much.