Port size and pumping loss. 2

[RodRico]

I'm adding pumping loss to my math models and have some questions for the experts.

My scavenge piston does nothing but draw ambient air in then push it through the main cylinder to flush exhaust gas and load the fresh air charge for intake (fuel is only added after both the intake and exhaust ports are closed).

I know the ambient conditions (at sea level and altitude), the air mass needs that needs to be moved, and the time over which it must be moved. From all of this, I calculate the required port area associated with a given pressure differential using the formulas below for choked and unchoked flow. I chose a conservative coefficient of discharge (C = 0.6) and low pressure differential (0.125 bar) when sizing the port area.

My questions are:

1) Since the air is flowing through two ports (intake and exhaust), do I have to double the differential ?
2) Is the pressure differential converted to isobaric work (Pressure * ΔVolume) the thermodynamic (sans friction) pumping loss ?
3) Isobaric work is positive as volume increases and negative as it decreases, so would the pumping loss be the difference ?

Thanks for any insight you can provide!

 

[BrianPetersen]

1) Yes, kinda. Keep in mind that your exhaust port is operating at elevated temperature and the exhaust process probably starts with a blow-down period, and the whole process is quite unsteady, and if that blow-down period isn't finished by the time the intake port opens then there may be a period of reverse flow in the intake port. When the time and pressure-wave effects start to be taken into account, this gets really complicated. Prof Blair has written books about two-stroke engine scavenging, which might be somewhat applicable to your situation.

2) Compressibility effects mean that this won't be exact.

3) I don't understand what you mean.

I've only paid much attention to four-stroke tuning, and in those, the pressure-wave effects on cylinder filling are extremely significant, much more so than simply treating the ports as steady-state ducts with frictional losses. The convoluted expansion chambers seen on high performance two-stroke engines suggest that the pressure wave effects are extremely significant there, too.

 

[RodRico]

Brian,

Good point on blowdown. I’ll make sure the product of port area and timing accounts for blowdown when I update the cam design for this version.

Unlike most two strokes, I’m not depending on exhaust back pressure to keep the air/fuel mix in the cylinder. I scavenge with air alone and only add fuel after the intake and exhaust ports close.

I’m doing the math model of port flow only to get in the ballpark. After the updated CAD model is done, I’ll run CFD on the ports. Even that will only be an approximation, but it should help identify any gross problems.

Rod

 

[RodRico]

Brian,

Blowdown turned out not to be too bad. Sizing the ports for everything else, worst-case blowdown only requires 3.4 degrees (of a 90 degree cycle) for cylinder pressure to fall to ambient pressure.

One reason it takes so little time for blowdown is that I'm almost fully expanded; The pressure drop to ambient when the exhaust port opens is only 0.9 - 1.2 bar. These figures are likely too low given an estimated 1.6 bar FMEP, but they're driven by the need to intake a lot of air for operation at 15,000 feet (it's a long story, but the intake air required to attain the desired compression temperature at altitude defines the distance between the intake and exhaust ports, and this in turn defines the expansion ratio. In other words, the compression and expansion ratios are the same at 15,000 feet while intake volume and thus compression ratio declines as altitude falls to sea level).

Rod

 

[Tmoose]

FWIW blowdown in a conventional high performance 2-stroke with Schnürle’s loop scavenging system, and active expansion chamber assisted scavenging and exhaust plugging needs to be around 27 degrees with exhaust open around 90 degrees.

I'm speculating everybody might benefit from good scavenging, which is much more than just a falling to ambient pressure. Schnürle’s scavenging system was originally developed for large slow-running stationary two-stroke diesel engines as used by the Deutz Company. In days gone by scavenging was sometimes known as flushing or rinsing..

It sounds to me like the return of slug of clean scavenging air via the exhaust plugging action of a nicely done expansion chamber would be of benefit even in a direct injection situation.

https://www.cycleworld.com/reflecting-on-exhaust-waves/

 

[RodRico]

Tmoose,

I'm not using loop scavenging. My engine is an opposed piston two-stroke using uniflow scavenging. You can see a discussion of this method in the paper at

https://www.mdpi.com/1996-1073/10/5/727/htm

. Note, however, that I don't use the "folded cranktrain," I use cams.

I don't see the benefit of back pressure in a uniflow scavenged engine that doesn't have any fuel in the scavenge charge.

Rod

 

[gruntguru]

I don't see the benefit of back pressure in a uniflow scavenged engine that doesn't have any fuel in the scavenge charge.

The usual benefit is free supercharge. (Not saying it would be feasible in your situation - with so many cylinders.)

je suis charlie