Exhaust gas mass flow rate

[Azmio]

Hi,

Is there anyone out there having an excel or simulation program which can calculate the exhaust mass flow rate going out of a cylinder for an engine with two 18-19mm diameter exhaust valve.

I know that it involves a lot of variables but I am more interested in term of the overall range in term of g/s.

 

[dcasto]

No need for an program. The mas out of the engine is exactly the mass going in. The pounds per hour of fuel plus the pounds per hour of the air.

The diameter has nothing to do with mass flowrate.

Are you looking for pressure drop, maximum flow for a given internal pressure?

 

[Azmio]

Dcasto

I am building an Excel spreadsheet to simulate a new combustion cycle. I wish that it is easy as what you're saying.

I am now stuck at the blowdown phase and I am quite concern that I dont have enough time to get rid of the exhaust gases.

Oh by the way, the duration of the blowdown phase depends on the exhaust valve sizing, lift and number of valves. That's why it is not as simple as the mass will go out due to mass in = mass out

 

[BrianPetersen]

Rather than trying to re-invent the wheel with an Excel spreadsheet, you will be much further ahead of the game by using an engine simulation program.

I've used Lotus Engineering. There's a freebie version that's full-featured but limited to a single cylinder engine, but this may be good enough for your needs, and likely more accurate than trying to fight with a spreadsheet.

Yamaha FZR400 exhaust valves are in that size range. (100cc per cylinder, 4 valves per cylinder, 56.0mm bore, 40.5mm stroke, revs to approx 14,000 rpm, approx 15 horsepower per cylinder in more-or-less stock form bit with a decent exhaust system on it.) If what you're trying to do is not dissimilar to that, you'll be fine.

 

[ivymike]

That's why it is not as simple as the mass will go out due to mass in = mass out

I assume you're not planning to perpetually accumulate mass in the cylinder, so the above still applies... unfortunately you don't have a handy way to calculate flow in, either.

As Brian notes, there are some programs that can help. Wave, Excite, GT Power, etc.

You could also benchmark some known valve designs to see how well they flow, and use an appropriate value based on that experience.

 

[Azmio]

Brian and Ivymike,

Do you have the link to the freebie? The number that I got for my mass flowrate is a way too small, I have tried to check for any unit conversion error but there is none.

I understand that it's much simpler to use wave or gt power but these tools are handy for conventional engine, my combustion cycle is not.

Part of the research is to build the analytical model. So far it has made me understand a lot about what's happening in the engine in every crank angle. On the other hand, it involves tons of math and the last time I toyed around with classical calculation for a cycle was during my undergrad 15 years ago.

By the way, do you have the average flow rate per valve for that FZR 400?

 

[BrianPetersen]

http://www.lesoft.co.uk/

Click on "Freeware" on the right side of the menu.

User interface of that program bites the big one, but the data file is a simple text file that can be edited with Notepad. I found it easiest to get the configuration how I wanted it even though it wouldn't let me enter the numbers correctly, then edit the data file directly by figuring out what number meant what. (Can't complain tho ... it's free.)

If your combustion cycle is not "conventional", this route might not work for you, because it's expecting a conventional 2-stroke or 4-stroke piston engine operating more or less on the Otto cycle.

I don't know the mass flow-rate per valve on my motorcycle engine. It could be calculated (approximately) based on intake air density, revs, estimated volumetric efficiency, etc., and like others have noted, the mass flow rate "out" has to be the same as the mass flow rate "in". Your calculation would be just as accurate as mine. This is my roadrace bike, and I'm limited by the rulebook for what's done to the engine, so it's pretty much stock, and it's irrelevant to me what the mass flow rate is. The engine runs, the carbs are jetted correctly, it makes normal power, and beyond that, "it is what it is". No point analyzing what the rules won't let me change anyway, so I've not bothered.

Conventional Otto-cycle exhaust systems depend strongly on the remaining pressure in the cylinder to force exhaust out the exhaust valves when they open, this is why the exhaust valves are always smaller than intake. If you're doing something out-of-the-box that changes this, standard analysis might not do what you want it to do.