# The Naked Scientists Forum

### Author Topic: At what rate is oxygen used in an average car engine?  (Read 13230 times)

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« on: 14/05/2008 13:40:27 »
Say 1.5 litre petrol at a continuous 3000 rpm ?

Further could said amount of O2 be injected to 'surround' the injected fuel?

#### Bored chemist

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##### At what rate is oxygen used in an average car engine?
« Reply #1 on: 14/05/2008 19:03:41 »
I'm not sure how the calculate the engine capacity but I guess it's the total displaced by the pistons. Half the time when the piston is going down it is pulling in air so that's 1.5L 1500 times a minute. That makes 2250 litres of air a minute of which roughly 20% or 450 litres a minute is oxygen.
I think most of that oxygen is consumed burning the fuel; having much more or much less air than you need would be wasteful.

I'm afraid I don't understand what you mean by the saecond question.

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #2 on: 14/05/2008 22:27:32 »
In a 4-stroke (am I right in thinking?) air is only drawn in a quarter of the time.
So this gives: 225 l/min, yes?

I'll try a different tack: Ignoring the size/speed of engine:

OK, I will try some maths :( ...

stoichiometric combustion ratio is 14.7:1
1lt weighs of petrol weighs 0.72kg

Say car gives 12km/litre (~36 miles/UK gallon)
- or 8.64km/kg of petrol

In fifth gear the car travels ~1.9km/min (70 miles an hour)

0.22 kg of petrol consumed in a minute

So 14.7 * 0.22 gives 3.23kg of oxygen needed per min.

Have I messed up somewhere?

The 2nd part of my question means:
Say all the air in the cylinder was inert except the said O2 above,
for instance recycled exhaust gas, could a 2nd injector that surrounded the fuel injector be used to put an oxygen cloud around the fuel vapour at the point of ignition?

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #3 on: 14/05/2008 22:43:15 »
No. BC is right about the quantity of air - the piston draws air every other revolution -  it takes only half the time for that revolution but it still takes in a cylinder's worth.
You could double your figures in the first sum.
The sevond (energy based) calculation is more likely to be right because you have included the efficiency - actual measurements involved.
With a lean burn engine, most of the fuel is burned anyway but concentrating the Oxygen could speed up the process, which would help efficiency, perhaps. But you would still need to be heating up the inert gas so that could work against your idea. The Diesel engine gets quicker burning because it all goes at once rather than having a 'flame front' moving down the cylinder. I think that partly accounts for the better efficiency. The PV diagram is a different shape for the diesel cycle.
The emission regulations require some unburnt fuel in the exhaust to keep the catalytic converter stoked up, I believe.

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #4 on: 15/05/2008 11:28:46 »
Thanks SC.
Do you have a reference PV diagram for spark ignition engines?
(I found one for diesel on wikipedia)

But, ignoring diesel for now:
If it was somehow possible to place a globule of fuel in the centre of a sphere of oxygen, that was then surrounded by an inert charge (say hypothetically pure CO2) centred in the cylinder & ignited - the flame front would expand centrally burn ALL the O2 - then transferring the heat equally throughout the charge.
Let's forget this expansion is confined by a cylindrical body for now: can work out the topology of the burn later.
The advantage would be, with CO2 being denser charge than nitrogen, there would be less potential for heat transference to the cylinder walls plus less charge needed for the same volumetric expansion. The exhaust with an idealised burn would only contain CO2 & a small amount of steam.
The hot exhaust could be cooled & dried and re-entered in the inlet manifold.
Only a small fraction of CO2+H2O would leave at the 'tailpipe': no cat needed.

I realise this is deeply hypothetical, but considering the complexity of modern engine systems is there any obvious physical constraints to make it infeasible?

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #5 on: 15/05/2008 13:42:35 »
http://webserver.dmt.upm.es/~isidoro/bk3/c17/Power.doc
This is a large word doc which has more than enough in it about the Otto Cycle etc for diesel and spark engine.
To get good efficiency, I think you need to take in cool air so you would need another heat exchanger on top of your normal radiator, inter-cooler and air conditioner .
But everything happens very fast - adiabatically -  so how would you get all the inert gases heated up if you didn't mix them with the Oxygen before injecting them into the cylinder?
It seems that your method wold involve all the expense of separating out the Oxygen in Air, only to mix it with another gas - unless you could find a process which produced Oxygen as a byproduct???
How about splitting water into H and O and using a Hydrogen Powered Car?
OH NO.  I feel a headache coming on.

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #6 on: 15/05/2008 14:17:25 »
I don't see that heating of the charge is basically any different from a normal spark ignition.
Here, taking a carburettor model, one homogeneous mixture of charge, oxygen & fuel droplets still needs time for the flame-front to expand from the initial point of the sparkplug.

I think the cooling circuit could be simplified. A reservoir could hold a limited amount of cooled CO2 for starting, which could also act as a gas coolant in the (traditionally, water) jacket around the cylinders.

Additionally, less heat might reach the walls as the higher density of CO2 over air would offer lower heat transference - with less initial charge for equal expansion.

Admittedly, a lot more heat would need to be removed in this semi-closed system & the time needed for this to be radiated away (or reused for another purpose) may well make the intermediate reservoir impractically large.

However, I'd like to see if it could be made to work!!

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #7 on: 15/05/2008 17:30:13 »
You need to heat the CO2 in order for it to expand. Heat transfer would be very slow - too slow to expect a mass of gas to heat up in a few tens of milliseconds. A flame front moves faster than conduction and still limits efficiency. The diesel process gets it all alight at once, virtually.

But, mainly, where are you going to get cheap oxygen from?

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #8 on: 15/05/2008 17:54:20 »
>> But, mainly, where are you going to get cheap oxygen from?
ANS: http://www.nda.ox.ac.uk/wfsa/html/u01/u01_009.htm
Bored Chemist put me on to this in an earlier post...

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #9 on: 15/05/2008 18:06:48 »
Am I right in thinking the rate of heat transfer is governed by specific heat capacity. Does this figure also define expansion?

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #10 on: 15/05/2008 22:25:37 »
Gases are very good insulators, you know. It would be too slow even through 20mm of metal! There would be a bit of convection through your CO2 but you want it pretty much instant, don't you.

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #11 on: 16/05/2008 09:02:35 »
From what SC says it seems coincidental that air represents such a suitable cocktail for an ICE (I suppose one could argue that we would have invented some other motor design if it wasn't!).

When I pictured a CO2 cooling system I thought of air cooled motorcycle engines...

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #12 on: 16/05/2008 16:43:38 »
Quoting myself:
stoichiometric combustion ratio is 14.7:1
1lt weighs of petrol weighs 0.72kg

Say car gives 12km/litre (~36 miles/UK gallon)
- or 8.64km/kg of petrol

In fifth gear the car travels ~1.9km/min (70 miles an hour)

0.22 kg of petrol consumed in a minute

So 14.7 * 0.22 gives 3.23kg of oxygen needed per min.

Have I messed up somewhere?

Oooops. Have just checked...

A litre of oxygen weighs 1428mg, so my calc predicts 2260 litres!!

This must be wrong as BC showed that if all the O2 is used only 450 litres are available every minute.

Can anyone tell me where I went wrong?

#### lyner

• Guest
##### At what rate is oxygen used in an average car engine?
« Reply #13 on: 16/05/2008 17:23:53 »
From what SC says it seems coincidental that air represents such a suitable cocktail for an ICE (I suppose one could argue that we would have invented some other motor design if it wasn't!).

Yes - I sort of implied that; a bit blinkered!
I guess there may be other N to O ratios that could perform better. We can't expect to be getting the optimum. Or maybe it doesn't matter too much. However, there are other considerations like how easy it would be to burn any fuel at all in a weak Oxygen mixture or how fiercely a fuel would burn in more Oxygen and could damage a an engine.
Designs have obviously been based on normal Air mixtures and I'd bet that not too much thought has gone into alternatives for piston engines.
Of course, rocket engines are in a different league. I can't imagine anyone proposing to include any extra inert component in a rocket propellant.

#### techmind

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##### At what rate is oxygen used in an average car engine?
« Reply #14 on: 16/05/2008 23:49:13 »
I'm not sure how the calculate the engine capacity but I guess it's the total displaced by the pistons. Half the time when the piston is going down it is pulling in air so that's 1.5L 1500 times a minute. That makes 2250 litres of air a minute of which roughly 20% or 450 litres a minute is oxygen.
I think most of that oxygen is consumed burning the fuel; having much more or much less air than you need would be wasteful.

But don't the cylinders operate at reduced pressure? So all else being equal, the above approach would overestimate the (atmospheric pressure-equivalent) volume of air.

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #15 on: 17/05/2008 12:57:09 »
Quote
But don't the cylinders operate at reduced pressure?
At what stage are you suggesting that this happens? Of course, during the induction stroke, there is less than AP but, with a well aspirated system  the pressure will be not be far below AP by the bottom of the stroke. And with a turbo . . . .

#### Bored chemist

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##### At what rate is oxygen used in an average car engine?
« Reply #16 on: 17/05/2008 15:53:39 »
peppercorn, where did you get the number 14.7 from?

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #17 on: 18/05/2008 14:05:42 »
peppercorn, where did you get the number 14.7 from?

Stoichiometric air-fuel ratio for gasoline given at:
http://en.wikipedia.org/wiki/Stoichiometric

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #18 on: 18/05/2008 16:28:04 »
Going back to specific heat capacity, could a well mixed combination of CO2 & O2 give the same resultant expansion rate in a cylinder as air?
ie, a nitrogen free engine...

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #19 on: 18/05/2008 21:24:43 »
What has expansion rate got to do with it? They all obey the gas laws and any engine would be designed with an appropriate compression ratio and cubic capacity to do an optimal job. I don't think the inert part of the fluid is very relevant. What counts is getting a rapid heating of the WHOLE mass of fluid and that, I think, implies good mixing of fuel and oxygen AND inert gases plus rapid combustion by one means or another.
Of course you could make an engine which worked with CO2 and some other mix of O2 and N2 might be better but it seems so much like standing up in a hammock. What's wrong with air? It's so readily available.

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #20 on: 18/05/2008 23:37:12 »
What has expansion rate got to do with it?

I don't know that's why I posted:

Am I right in thinking the rate of heat transfer is governed by specific heat capacity. Does this figure also define expansion?
a couple of days ago - as I was struggling with the concepts then (as now!).

...What's wrong with air? It's so readily available.

I thought:
An engine without nitrogen would have no chance to create NOx's.
Also a 'heavier' gas should, on the face of it, soak up the thermal energy more readily, turning it into expansion - hence less wasted energy & potential for removing cooling system completely.

CRAZY I'M SURE....

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #21 on: 18/05/2008 23:53:51 »
Quote
Also a 'heavier' gas should, on the face of it, soak up the thermal energy more readily, turning it into expansion - hence less wasted energy & potential for removing cooling system completely.
That doesn't quite make sense to me. Why would it work that way?
The bit about Nox's could be relevant tho. But there could be a problem with CO being formed?

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #22 on: 19/05/2008 08:21:43 »
Quote
Also a 'heavier' gas should, on the face of it, soak up the thermal energy more readily, turning it into expansion - hence less wasted energy & potential for removing cooling system completely.
That doesn't quite make sense to me. Why would it work that way?
The bit about Nox's could be relevant tho. But there could be a problem with CO being formed?

#### peppercorn

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##### At what rate is oxygen used in an average car engine?
« Reply #23 on: 19/05/2008 09:44:15 »
Quote
Also a 'heavier' gas should, on the face of it, soak up the thermal energy more readily, turning it into expansion - hence less wasted energy & potential for removing cooling system completely.
That doesn't quite make sense to me. Why would it work that way?
The bit about Nox's could be relevant tho. But there could be a problem with CO being formed?

Well, that's the thing - I must have a complete misunderstanding of how gases (or any material for that matter) act when heated.

I though heat would be absorbed by a denser gas more than a lighter one.
-Although, now, just picturing it in my head, I would also expect that the time taken for absorption with a dense would be longer - ie, too long with CO2 for the revs of an ICE.

I suppose I  have been getting two very  simple parameters muddled up: heat absorption by a molecule and heat transfer speed...

#### lyner

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##### At what rate is oxygen used in an average car engine?
« Reply #24 on: 19/05/2008 13:20:07 »
I never paid much attention to the Thermodynamics I did at Uni, I'm afraid, so I can't be too dogmatic about it but there are a lot of websites about basic gas laws and heat engine cycles.
Just search on Gas Laws and Gas Constant; you are bound to find something for your particular level of understanding - then work upwards!
Good luck.

#### The Naked Scientists Forum

##### At what rate is oxygen used in an average car engine?
« Reply #24 on: 19/05/2008 13:20:07 »