dubulup

20B with a GT42R

Nihilanthic

Oh, sure, but the car would be at least 35K new, if that were the case :x

I'd rather the 13b or the supposed new 16b to keep it affordable :P

KevinK2

when using applets for sizing compressors, reduce VE to account for back pressure from turbine. 85% is a good starting point.

Found this combustion chamber surface area analysis I did, comparing with a 4 cyl 2.6L engine. The TDC and BDC conditions are very accurate. The "new" area for the FD is less than I figured, as I recall.

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thermal inefficiency

The rotary generally wastes some fuel. It has a high BSFC, vs piston engines.
A good portion of fuel energy goes into the cooling system as heat, during the
power and exhaust strokes. This is due to a higher surface to volume ratio
of the combustion chamber, vs piston engines.

How much higher? I ran some numbers to check this difference in
surface areas for the same displacement piston engine (2.6L) and rotary (FD).

Comparo follows with '2.6L' 944 4 cyl. I assumed spherical comb'n chamber,
enlarged for same FD 9.0 compr ratio. adjusted bore and stroke from oem 2.5L,
to get 2.6L with same bore/stroke ratio. The 944 is a convenient comparo, as it
fires 2 pistons per rev, and the rx7s fire 2 rotor faces per crank rev.

For FD rotor, R=105mm, and w=80mm. 40 cu-in face displacement. Determined
rotor face contour radius is 7.67", and found larger radius for 5 cu-in chamber at
TDC, then found radius for 40 cu in displacement from there. Also repeated calc's
using flat TDC chamber assumption ... with about same results. I think the
approximation of the chamber in the housing as a constant radius should be
close enough for this comparo. Here are calc'd heat exchange areas, in sq inches:

944 ___ FD __ % more FD

Total @TDC
26.9 __ 51.0 __ 90%

Total @BDC
66.8 __ 86.1 __ 29%

NEW area @ BDC
39.9 __ 61.5 __ 54%

When things are the hottest near TDC, the area issue is the worst. Also a time factor.
The rotary stroke is during 3/4 crank rev. while 944 is only 1/2 rev. The rotary will
allow more time for heat exchange, vs the 2.6L 944 example, at same rpm.

I recall the winning 4 rotor LeMan car had the internal sufaces thermally coated
to minimize any heat loss.

KJK 11-26-02

Eggie

As Speedturn noted for the Wankel, the exhaust port opens much earlier than in a 4-stroke piston engine. Same holds for a 2-stroke.

KevinK2

exhaust port event specs per fsm's, FD vs 04 Mazda6 V6:

FD -- opens 75 btdc, closes 48 atdc, duration 303 deg
M6 -- opens 71 btdc, closes 17 atdc, duration 268 deg (check clearance not spec'd)

question is, is the FD spec based on crank degress, like piston engines? if so, crank degress for the rotary are 50% longer than the cycle degrees, defined by rotor rotation. This would mean, in rotor degrees, opening would be 50 btdc, and close 32 atdc, for non agressive port opening timing.

Ignition timing seems to be crank angle based, mabe port events, too. I'm not sure about port timing definition basis, shaft angles vs rotor angles.

Eggie

Thanks for that reply, Kevin. When I posted, I accepted that the Wankel's EO timing was WAY earlier than a piston's. However, I was thinking of a cam's commonly specced .050" number, not the absolute EVO timing. Oops!

Anyone have any area-time graphs they're willing to share?

Nihilanthic

a VE% over rpm graph would probably be a lot more useful, lol.

anyone got that? That would also help for plotting on a compressor map...

Though, unfortunately, I still dont know how to do it just by rpm... ugh.

rotarygod

iTrader: (1)

One thing that can get confusing with rotary port timing is that the opening and closing timing points are not referenced to the same top dead center. It sounds weird but top dead center is not the same for the intake stroke as it is for combustion and the timing numbers reflect this. It is actually a little confusing until you actually see it using a degree wheel on an actual engine.

KevinK2

Yup, part of the shared chamber surfaces deal. Two tdc positons for ea rotor face (horizontal, L and R), and two bdc positons (vertical, T and B).

Based on yaw's site, port timing is based on e-shaft (crank) degrees. It takes 270 e-shaft degs rotation to go from TDC to the next BDC position for a face, which reflects one of the 4 strokes (vs 180 degrees for a piston engine).

http://www.yawpower.com/dectech.html

If a valve closed about 1/2 way down a pistons' stroke, that would be about 90 crank degrees. If a port is closed 1/2 through a rotary's stroke, that would be about 270/2 = 135 degrees e-shaft rotation.

Main point is the port timing published is based on 270 shaft degrees per stroke, so 75 bbdc for a rotary port event is comparible to 50 deg bbdc for a boinger event, which is not radical. In fact, there is little if any overlap in intake & exhaust cycles cycles for a stock rotary, unlike a piston engine, especially a 2-stroke.

correcting "btdc" to "bbdc" on my prior post (my mistake) .........

> exhaust port event specs per fsm's, FD vs 04 Mazda6 V6:

> FD -- opens 75 bbdc, closes 48 atdc, duration 303 deg
> M6 -- opens 71 bbdc, closes 17 atdc, duration 268 deg (check clearance not spec'd)

Comparing duration in the case above, the exhaust is open 268/720 = 37% of a the M6 piston engine cycle (would be more at zero checking clearance ... ie valve seated), and 303/1080 = 28% of a rotary chamber cycle.

This is why I say most of the thermal efficiency losses in the rotary are due to the surface area thing, not the port timing.

EScalade

A Wankel engines combustion chamber is square.

This is definately not ideal because as fuel/air enters the combustion chamber it cannot naturally fill an a natural (circular) motion.
Valves do not cause as much obstruction as poor chamber shape.

BATMAN

I'm surprise that Jimlab hasn't chimed in yet.

I like to hear his thoughts on this topic.