Bore, Stroke, Displacement

[whitestivo]

Can someone help me with the following?

Once you calculate displacement for an engine, I'm trying to calculate how much air is taken in that displacement per minute

If I have a 1.8L displacement, am I correct if I'm doing 1500rpm (at idle), that only half the shafts spin per cycle, so I'd divide my displacement by half to 0.9L, then multiply by 1500 to get total displacement per minute of 1350litres?

Then my question is when the mist of fuel is injected, roughly what percentage of the combustion mix is fuel and air, I'm thinking something like 10% would be adequate, so that would leave me with 1215lites per minute of air? It seems like a lot.

I know my stivo sucks huge wallops of air through the CAI as I can hear it, but really that much?

Information would be greatly appreciated.

whitestivo

[DJKOR]

I'm not good with the maths and all, but from what you have written, that seems fairly close. I remember reading the HowStuffWorks article on the Bugatti Veyron and found that the Veyron needs to consume around 45,000 litres of air per minute at maximum power. Bearing in mind, it is a larger engine with more power, but it goes to show the that air intake is quite a bit.

[chicaboo]

Hey Phil,

I can't answer your questions on the maths, but I can tell you why looking at CFM is really useless for NA til you rate it redline (8200rpm), and not 1500rpm.

The engine will suck in the same volume of air every cycle no matter what the rpm is. It just happens faster as the engine speed increases.

So you really want to know how much air the engine can suck each cycle based on it's swept volume. From that you can multiply to work out CFM at redline or any rpm.

Basically CFM doesn't change as far as the engine is concerned, but it changes at the IM, TB and in the CAI and everywhere else I guess. Sorry for the circular logic. :P

Atleast from that you can work out the ideal size for a TB, and CAI at max rpm.

Cheers, Gav.

[Hiro]

Remember that an internal combustion engine is just a glorified air pump. So the fact that it consumes ridiculous amounts of air is not surprising at all.

[whitestivo]

Thanks guys, can always rely on great answers. Working on Djkor's numbers, I'm about right with my calc

Was I definitely correct about the half the shaft spin per cycle, halving the rpm?

Gav, I lost you at IM, TB? What are they?

I still find the numbers quite staggering.

whitestivo

[DJKOR]

Was I definitely correct about the half the shaft spin per cycle, halving the rpm?

That's correct. Because in a 4-stroke, 1 cycle consists of intake and compression, the next cycle is for combustion and exhaust.

Okay, well I took it one step further for a little comparison. In my previous post, I mentioned the HowStuffWorks article. Using the following, I made some figures on my own:

* 1,000 horsepower is equivalent to roughly 2.6 billion joules per hour. A gallon (3.8 liters) of gasoline contains 132 million joules, so a 1,000-hp engine has to be able to burn just over 20 gallons of gasoline per hour.

* However, car engines are only about one-quarter efficient -- three quarters of the gasoline's energy escapes as heat rather than as power to the wheels. So the engine actually has to be able to burn at least 80 gallons per hour, or 1.33 gallons (5 liters) per minute.

* Let's convert over to metric. Gasoline requires about 14.7 kilograms of air to burn 1 kilogram of gas. Air weighs 1.222 kilograms per cubic meter at sea level. A gallon of gasoline weighs 2.84 kilograms. So the engine has to be able to process 2.84*1.33*14.7 kilograms of air per minute, or roughly 45 cubic meters of air per minute. That's 45,000 liters of air per minute.

* If a V-8 engine is turning at 6,000 rpm, it can inhale a total of 24,000 cylinders' full of air per minute. If it needs to inhale 45,000 liters of air per minute, it works out to roughly 2 liters per cylinder-full. That's a 16-liter engine.

Now everything before the italics still stand fairly correct. The use of lift on the 2ZZ screwed up the rest of the calculations.

So if we say the 2ZZ puts out 189 hp @ 7,600 RPM, then:

- 189 hp is equal to about 500 million joules per hour

- If 3.8 litres of gasoline contains 132 million joules, then to produce 189 hp you need 14.4 litres per hour

- As an engine is only about 1/4 efficient, you therefore need 57.6 litres per hour or 0.96 litres per minute

- One litre of gasoline weighs 0.75kg per litre, therefore the engine has to process 0.75*0.96*14.7 kilograms of air per minute

- This roughly equates to 8.66 cubic metres of air per minute or 8,666 litres of air per minute

- If a 4-cylinder engine is turning at 7,600 RPM, it can inhale 15,200 cylinders' full of air per minute

- If it needs to inhale 8,666 litres of air per minute, then it works out to be 0.57 litres per cylinder-full, which equates to a 2.28 litre engine.

^^^ So I guess the use of lift gives the 2ZZ the capabilities of a 2.28 litre engine running at 7,600 RPM.

[chicaboo]

That articles going in another direction, as it's looking at the amount of air required to achieve a certain amount of power with a certain volumetric efficiency.

If you disregard power, and how efficiently the engine can intake and discharge air, you can still work out the amount of air it pumps regardless of the above.

Things like porting and larger valves won't make the combustion chamber hold more oxygen, but it will become more efficient at using it, so it makes more power.

The article is still interesting though. At a guess I would've thought the Sportivo motor was as efficient as a 2.4L motor while it's on the big cam. Not far off.

Phil:

TB - Throttle Body

IM - Intake Manifold

VE - Volumetric Efficiency

Cheers, Gav.

[whitestivo]

DJK, I reckon I've read your post about three times to understand it, good work

I believe I can disregard effiency as that concerns power output, the volume of air must be the same as the displacement and rpm are the same (which I think is what Gav is saying)

thanks guys, I'm confident now that I got it close to right.

whitestivo

[Blind Kid Seeks]

great thread guys... very interesting read

now can you summarise for the layperson.

[Hiro]

I believe I can disregard effiency as that concerns power output, the volume of air must be the same as the displacement and rpm are the same (which I think is what Gav is saying)

Volumetric efficiency cannot be ignored, as it is the ratio of air actually drawn in compared to displacement/cycle. An engine does not necessarily draw in a full displacement's worth of air every cycle, as things like flow restrictions, valve overlap, back pressure, scavenging etc. A supercharged engine (either via a centrifugal turbocharger, centrifugal supercharger or positive displacement blower) will have a higher volumetric efficiency, often greater than 1, as more air is being pushed into the engine due to the air being compressed ("boost"). N/A engines can have VEs greater than 1, usually via things like lift, or intake speed is increased so that air flows in quickly and then not allowed to escape, thus creating partial boost despite not using a turbo/supercharger.

Think of it as a train. The train has 60 seats in a carriage (displacement), and the doors remain open for 30 seconds at a station (valve duration), and the fare each passenger pays is the power created. For a VE of 1, 2 people will enter the carriage every second, thus completely filling the carriage at normal capacity (naturally aspirated). If the flow into the carriage is restricted (ie posts on the platform, small doors etc), say only 1.5 people can enter per second. Thus, the carriage will only have 40 seats filled before the door closes (0.667 VE). If there is an outside force pushing the people (turbo/supercharger, think the "pushers" they have on the Japanese metro), you can get more people in before the door shuts, even though they don't all have seats to sit in (and thus are squashed in, creating extra revenue or "boost"). Or, if flow is exceptionally good without the pushers, lots of people will get into the carriage, but find out that they have nowhere to sit. If the doors shut before they can get back out, the boost situation will be recreated.

[chicaboo]

Oh nose Habib! Now you've gone into the world of volumetric efficiency he will never be able to work out his max CFM at 8200rpm. :P

Props for the good analogies on VE BTW.

Cheers, Gav.

[xoom]

well here is a good read off newcelica

http://newcelica.org/forums/showthread.php?t=256391

it was completely useful to me due to the path i took but some might want to educate themselves ;)

Bill

[chicaboo]

well here is a good read off newcelica

http://newcelica.org/forums/showthread.php?t=256391

it was completely useful to me due to the path i took but some might want to educate themselves ;)

Bill

Even the stock USDM spec motor at 8500rpm still has 86.8% VE. Not sure how VE changes at 8200rpm, but with CAI and ported headers/catback, let's assume our cars have 85~90% VE.

Interesting to note how easily the 2ZZ approaches (and passes) 100% VE. I thought it would be pretty high, but that's amazing!

Anyhow, that's why I was saying Phil didn't need to worry about anything but CFM at peak rpm. There's probably only up to 10% change in his figures when you throw VE into the mix.

Awesome reading, thanks guys.

Gav.

[SuperDave]

Then my question is when the mist of fuel is injected, roughly what percentage of the combustion mix is fuel and air, I'm thinking something like 10% would be adequate

Stoichiometric is about 14.7:1 with petrol. The ECU mapping will try and keep close to that, so doing your maths based on the figures a dyno graph gives, about 13:1 might help you along.

At 1500rpm, and depending on load the injectors are only at about 3-5% duty cycle. The Sportivo comes with 330cc/min injectors, so each minute there is 16.5cc/min (assuming 5% duty). Based on the AFR from a dyno I'd guess that this results in 215cc/min of air. This is only per cylinder, so 4 times that is about 860cc/min of air. I maybe wrong here, but thats my thinking on it

[TRIPPER]

Very good info guys. We need more in-depth threads like these!!

[Hiro]

Then my question is when the mist of fuel is injected, roughly what percentage of the combustion mix is fuel and air, I'm thinking something like 10% would be adequate

Stoichiometric is about 14.7:1 with petrol. The ECU mapping will try and keep close to that, so doing your maths based on the figures a dyno graph gives, about 13:1 might help you along.

At 1500rpm, and depending on load the injectors are only at about 3-5% duty cycle. The Sportivo comes with 330cc injectors, so each pulse is 16.5cc (assuming 5% duty). Based on the AFR from a dyno I'd guess that this results in 215cc of air. This is only per cylinder, so 4 times that is about 860cc of air. I maybe wrong here, but thats my thinking on it

Big flaw there. Injectors aren't rated in cc, they're rated in cc/min, but most people just drop the minutes. So a 2ZZGE injector doesn't have a 330cc capacity (slightly smaller than a can of Coke), it can flow 330cc of fuel per minute. And the amount of fuel that enters the cylinder isn't necessarily the amount that gets sprayed by a single pulse, since a lot of cars run wasted fuel (they inject even when the valve isn't open, ie on cylinders that are in the compression/exhaust cycle). To work it out, you have to calculate how much fuel total is being injected for a full engine cycle (ie full intake/compression/ignition/exhaust strokes for each cylinder), and then dividing by the number of cylinders to work out an average.

Edited December 29, 2008 by Hiro Protagonist

[SuperDave]

Big flaw there. Injectors aren't rated in cc, they're rated in cc/min...

Corrected my units so it's now flow rate and not volume :P

I'm not saying I am right, as I am not an engine designer. It is always good to learn something new and to get ones brain thinking. I was just giving another possible method of determining the amount of air required.

Based on data that can be collected, in my method, and compared with the above mathematics, at 8000rpm I am within 10%.

At lower loads/speeds it is out by a lot, which could be due to the efficiency factor; or my method is just crap :P

[Hiro]

Big flaw there. Injectors aren't rated in cc, they're rated in cc/min...

Corrected my units so it's now flow rate and not volume :P

I'm not saying I am right, as I am not an engine designer. It is always good to learn something new and to get ones brain thinking. I was just giving another possible method of determining the amount of air required.

Based on data that can be collected, in my method, and compared with the above mathematics, at 8000rpm I am within 10%.

At lower loads/speeds it is out by a lot, which could be due to the efficiency factor; or my method is just crap :P

Cars almost never run stoich either, they lean out at low loads to decrease consumption, and rich up at high speeds to stop overheating (extra fuel cools the combustion), so that can affect numbers significantly...

Edited December 30, 2008 by Hiro Protagonist

[SILVABULLIT]

And Phil you need to know all this WHY????

Have you got some black mail over you wife/partner to spend lots of money on the Stivo

[xoom]

And Phil you need to know all this WHY????

Have you got some black mail over you wife/partner to spend lots of money on the Stivo

if this is true ..... come down and see us @ kebab performance ....... u'll give us a stock stivo and in return u'll get a 12 second weapon ;) ;) ;) ;) very unhealthy on the bank account though :lol:

[Hiro]

if this is true ..... come down and see us @ kebab performance ....... u'll give us a stock stivo and in return u'll get a 12 second weapon ;) ;) ;) ;) very unhealthy on the bank account though

His bank account obviously, not yours :P

[xoom]

if this is true ..... come down and see us @ kebab performance ....... u'll give us a stock stivo and in return u'll get a 12 second weapon ;) ;) ;) ;) very unhealthy on the bank account though

His bank account obviously, not yours :P

my bank account should hopefully get nice and healthy from the transaction ;) ;) ;) lol

u just let me know phil wen ur ready to turn ur beast into a pocket rocket that will make all the QLD boys jealous ;) ..... even the ones that drive black commonwhores :P