Fuel consumption on 4 stroke engines

[scoff]

Hi everyone,

I've got a question about four stroke engines. I would like to know how much a cilinder is filled with the fuel/air mixture during the intake stroke. I tried to calculate it but somehow my fuel usage is far to high. I tried the following:

4 stroke 3.0 litre V10 engine
13.5 to 1 air/fuel mixture

In a four stroke engine all cilinders have 1 intake-stroke every 2 rounds, so at 10.000rpm each cilinder has 5.000 intakes per minute. When the full cilinder (for instance 300cc) would be filled in an intake-stroke there would be 5.000 intakes x 300cc x 10cylinders = 15.000 litres of air/fuel mixture going in and out of the engine. at a 13.5 to 1 air/fuel ratio that would be 1034 litres of fuel per minute. when you travel about 5km's in a minute that would be a very rich engine of about 200 litres / 1 km instead of a real racing engine around 1 / 1.5.

So i guess i'm making a very big mistake, or during the intake a 300cc cylinder will not be filled with 300cc air/fuel mixture of 13.5/1. I guess it is then limited by the amount that can get through the intake valves or/and is blocked by the remaining exhaust. If so is there a way to calculate how much % of fresh fuel and air gets in the cylinder each intake-stroke?

Can someone help me out?

[Rubinho]

I think your maths are a bit out.

Each cylinder is 3/10=0.3l (300cc but let's keep SI)

Air:fuel of 13.5:1 means that each cylinder gets (1/13.5) * 0.3 = 0.02l of fuel

5000 * 0.02 = 100l fuel per minute

[phoenix]

Fuel consumption is related to bhp, so it is probably better to consider the fuel consumption per minute per 100 bhp, for example, rather than to engine capacity.

100 bhp requires around 11.5 lbs of air per minute and therefore about 0.85 lbs per minute of fuel at an A/F ratio of 13.5:1. This works out to about .54 litres of fuel per minute. It should be noted that the air/fuel ratio is by weight, NOT volume - so the previous calculations are seriously wrong!

If a V10 3 litre engine produces 300 bhp it will require 1.6 litres of fuel per minute. If it produces 900 bhp it will require 4.8 litres of fuel per minute.

Hope that helps....

[Rubinho]

Of course, by mass!

I was so concerned with the errors in the maths of the OP's calculations I never even stopped to consider that! I thought 100l/min was still out by an order of magnitude but I couldn't figure out where it came from!

His was 2 orders of magnitude out so he was "wronger" than I was!

[phoenix]

No worries!!

But can you imagine how many laps an F1 car would manage before refueling at 100 litres per minute?

[ian_w]

Your calcs have all gone a bit awry.

First assumption is that the cylinder is 100% full of air/fuel mixture, for a race engine values can reach 120% but for this example we will assume 100%.

10000 rpm = 5000 intake events / min
Displacement = 3 litres
Air volume flow = 5000 * 3 = 15000 l/min or 15 m3/min ( cubic metres per minute )
Air mass flow = 15 * 1.2 = 18 kg/min ( air density = 1.2 kg/m3 )

Normal max power AFR is closer to 12.5 rather than the 13.5 stated, I will use 12.5 in this example.

Your calcs went wrong at this stage, AFR is mass based not volume based!

Fuel mass flow rate = Airflow/AFR = 18 / 12.5 = 1.44 kg/min

Fuel volume flow = mass flow / density = 1.44 / 0.72 = 2.0 l/min
( density petrol = 0.72 kg/litre )

[ian_w]

Forgot to say:

Pheonix's calcs on fuel flow are pretty good.

A petrol engine usually consumes about 280-300 g/kW.h ( grams per kilowatt hour ) of fuel, or around 220 g/hp.h at full power.

For Phoenix's 100 bhp example

Fuel flow = 220 * 100 = 22000 g/h = 22 kg/h = 0.367 kg/min = 0.81 lb / min

[phoenix]

I agree that cylinders filling can be over 100% - even 120% - but that will be at max Ve which will only happen at peak torque rpm. Achieving over 100% cylinder filling at peak power is very, very hard to achieve.

Thanks for the compliment about my numbers. I was using ballpark figures only and with plenty of rounding to give easy to read numbers. Even so your .81 lbs/ min is much closer to 0.8 than my calculations which rounded to .85 lbs/min. I will check my calculations in the morning!

[scoff]

Tnx a lot everyone especially Ian.

After reading it twice I understood it totally, very good explenation Ian!

[scoff]

Originally Posted by Rubinho

I think your maths are a bit out.

Each cylinder is 3/10=0.3l (300cc but let's keep SI)

Air:fuel of 13.5:1 means that each cylinder gets (1/13.5) * 0.3 = 0.02l of fuel

5000 * 0.02 = 100l fuel per minute

Rubinho you forgot to get back to the 10 cylinders and then you reach the same 1000liters as I got. but i used 13.5/1 and used 14.5 therefor to calculate.

[R59]

With all of those figures in hand, does that allow you to calulate how many laps your F1 car will be able to do before it requires a trip to Tesco's?

These calcluations are all based on fully open throttles.

I for one, don't tend to run with the right boot hard down for 100% of the lap. I know people who have tried, and then taken their cars home in small unrecognisable pieces afterwards.

Also, are those calculations / basic figures / worked out at sea level with a barometric pressure of 1000mb?

I guess the only engines that could come close to meeting the ramblings of a mathamatician's pencil are those in a F1 Offshore Powerboat? FLAT CHAT FOR HOURS!

Rob

[phoenix]

Originally Posted by racing59

With all of those figures in hand, does that allow you to calulate how many laps your F1 car will be able to do before it requires a trip to Tesco's?

These calcluations are all based on fully open throttles.

I for one, don't tend to run with the right boot hard down for 100% of the lap. I know people who have tried, and then taken their cars home in small unrecognisable pieces afterwards.

Also, are those calculations / basic figures / worked out at sea level with a barometric pressure of 1000mb?

I guess the only engines that could come close to meeting the ramblings of a mathamatician's pencil are those in a F1 Offshore Powerboat? FLAT CHAT FOR HOURS!

Rob

Of course it is very difficult to calculate fuel consumption on a race circuit without more information. The F1 teams of course know how much throttle is being used, and the fuel flow, at any instant in a lap, so they know how much their engine will use under race conditions and can allow in their calculations for changes in barometric pressure and temperature.

I was not trying to say that my calculation based on maximum bhp, obviously at full throttle, were representative of actual fuel consumption in a race, merely that as the previous calculation showed the consumption to be 100 litres per minute, so you could probably see that getting a couple of laps out of a tank of fuel would be a struggle!

My calculations were based on standard dry air temperature and pressure, i.e. 273.15 degrees Kelvin and 1 Kg/square centimetre. Obviously at higher temperatures or at lower pressure due to higher altitude the density of air decreases, so less fuel is needed.

However, as my calculations were dealing in BHP only they are valid at any pressure or temperature as the same volume of air and fuel are required to produce the same horsepower at any temperature and pressure.

As we all know, engines produce less power at altitude and at higher temperatures, because the air density is lower. In these situation the engine requires less fuel, so consumption on an F1 engine would be lower.

I agree that power boats must come closest to the 'perfect' fuel consumption calculations if the temperatures are steady, but I know a throttle man who would argue that without him his drivers would get nowhere - except to the bottom of the ocean pretty quickly! Power boats have varying loads on the propellor(s), which effects fuel consumption and the throttles are being adjusted pretty much all the time.

[ian_w]

Indycars on a high speed oval can run over a speed range of only about 300rpm and at full throttle for a whole tank of fuel ( assuming no yellow flags ) - thats pretty close to running continuously at peak power!

Fuel consumption is around 2.5 mpg ( on methanol ), which gives range of 75miles on the 30 gallon tank. Average speed can approach 220 mph so they need to fill up approx every 20 minutes.

Racer59: Calcs are done at 25degC and 101.3 kPa pressure. Different temps and pressures change air density from the quoted value of 1.2 kg/m3. A good book on Thermodynamics can tell you how to calculate this - I wont bore you with more maths!

[MountainRunner]

You could probably estimate to be on full throttle 80% to 85% of a lap, or check the throttle position data log and go from there.

[Chucky]

www.efi101.com

Do the course. They have all the answers.