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Intake Air Temp and Pressure vs Economy 

 

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 Post subject: Intake Air Temp and Pressure vs Economy
Posted: Mon Oct 03, 2005 10:53 am 
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Hi all.

I've seen round the forums for a while a few mis-truths when it comes to economy of your vehicle. One I have seen, or mayde it's just a general mis-conception, is that getting cold air will increase your fuel economy.

This is not the case based on the following reasoning.

We all know that if you get colder air into the engine, you increase the engine power by virtue of the fact that the air density is greater and hence, you get more air and fuel into the cylinder.

In trying to get the most power out of your engine, you need to make it as easy for the engine to breathe as possible, and you need to pack as much air into each cylinder as possible.

However, in trying to get the best economy out of your engine, you are solely concerned with making it as easy as possible for the engine to breathe. By contrast to getting the most power out of your engine, in getting the most economy, you should not al all be concerned with how much air you can pack into the cylinder. If you need 15litres/hour of fuel @ 100km/h, you need 220litres/hour of oxygen @ 14.7:1 ARF and since oxygen makes up only 21% of the air, you need 1050litres/hour of air.

This is controlled by the throttle. So, let's say you need 5% throttle to maintain 100km/h based on the above figures. If you decrease the temp of the air, you increase the density which gives you more power which is irrelevent because all this means is that you need to close the throttle to maintain 100km/h.

And therein lies the reason why cold air does not increase fuel economy. One of the biggest variables that impacts on economy is what's knows as pumping losses. The engine's trying to suck x amount of air in, but the throttle is restricting that air flow, so the engine has to work harder to get the amount of air it needs.

By increasing the density of the air and closing the throttle, you are making it harder for the engine to do its work, not easier, thereby making the engine use more fuel.

The answer? Hotter air give you better economy. This decreases the air density meaning you need a wider throttle opening to deliver the right amount of air to the engine. This is only effective however if you can adjust the spark to properly match the intake air temp, as a hotter, less dense air requires more time to burn and hence you need more spark advance.

The other thing that will decrease fuel economy is getting the air from a low pressure area off the car, such as the bumper or the bonnet. Both these areas have air passing over them @ your road speed in nill wind conditions. This has a venturi effect on any hole the air passes over and sucks air out of that hole thereby creating a low pressure area. Getting air from a high pressure area of the car makes it easier for the engine to suck air in and while it means you need a smaller throttle opening thereby increasing pumping losses, this is offset by virtue of the fact that air higher than atmospheric pressure is trying to force its way into the engine.

Incidentally, grabbing air from a low pressure area also decreases your engine's peak power @ speed. This will not be noticed on a dyno, but wind your car up to 150km/h + on a track, or even 100km/h on the freeway, and you engine will be putting out less power than on a dyno.
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Posted: Mon Oct 03, 2005 11:45 am 
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i find it hard to comprehend that a higher throttle percentage will return better economy than a smaller throttle percentage.

 

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Posted: Mon Oct 03, 2005 12:21 pm 
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With the price of fuel this days, I decided to remove the tickford snorkle & let the airbox suck in warm/hot air from the eng bay a week & 1/2 ago.

I'm well aware of tense air and its effect on fuel.

I've also notice my tail pipe has change colour from black to dark brown almost like rust in colour.
I've come to the conclusion that the eng was sucking in to much tense cold air that the standard ECU can only advace the timing so much. there for, access un-bunt fuel is blowing out as black silt.

performance? can't tel the difference.

economy? slightly better then before.

 

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Posted: Mon Oct 03, 2005 1:10 pm 
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prydey wrote:
i find it hard to comprehend that a higher throttle percentage will return better economy than a smaller throttle percentage.


Get a straw and suck through it. Easy huh?

Now close off the end of the straw and suck through it. You'll find it's much harder to suck the same amount of air through a smaller opening and you need to use more energy to do so.

Also, everyone knows that a 2 1/2" exhaust flows more air and allows the engine to breathe better and, coupled with extractors, allows better fuel economy. If smaller throttle openings allowed for better economy, then smaller exhaust systems would also allow for better economy and people would put 1/2" dia exhaust systems on their car.

You are thinking in the wrong way. Your line of thought is that if you close the throttle more, less air gets in and hence you get better economy. By doing this, it's like closing off the end of that straw even more, making it even harder to suck the air in.

If you don't believe me on this, do some research into the economy of aeroplanes @ different altitudes. You'll find that the higher you fly, the more economical it is and one of the reasons for this is if you fly high enough, you end up flying @ full throttle (in a piston engined aircraft) because the air is so thin and this provides next to no restriction caused by the throttle.
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Posted: Mon Oct 03, 2005 3:43 pm 
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so if i thrash my car rather than drive normally it will use less fuel as i am opening the throttle wider more often. throttle doesn't directly control the air intake it controls the fuel intake. the increase in rpm increases the amount of air sucked in. more throttle = more fuel = less economy.
your straw sucking example would back up the theory that larger intake would increase economy not the opposite as you would have us believe. with less restriction coming in the engine is not working as hard to get air in as opposed to a more restrictive intake. it normally also comes with a power increase as the engine is more efficient.

and i ain't going up in a piston engine aircraft any time soon either to test that theory. i'll take your word on that.

 

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Posted: Mon Oct 03, 2005 4:14 pm 
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prydey wrote:
so if i thrash my car rather than drive normally it will use less fuel as i am opening the throttle wider more often. throttle doesn't directly control the air intake it controls the fuel intake. the increase in rpm increases the amount of air sucked in. more throttle = more fuel = less economy.
your straw sucking example would back up the theory that larger intake would increase economy not the opposite as you would have us believe. with less restriction coming in the engine is not working as hard to get air in as opposed to a more restrictive intake. it normally also comes with a power increase as the engine is more efficient.

and i ain't going up in a piston engine aircraft any time soon either to test that theory. i'll take your word on that.


Your taking everything completely out of context and adding complexity to a very simple concept.

To put your theory, and attempt to debunk my explanation, on thrashing you car to get more economy to rest, let me explain this.

F = ma. Or, Force = Mass x acceleration. If you thrash your car, and accelerate at a faster rate, since your mass stays the same (actually, according to Einstein theory of relativity your mass will increase), the force must be greater. More force = more fuel.

I am talking about a situation where the engine's load, and revs are relatively constant, like on freeway driving. In that situation, you can still use the equation F=ma. Your mass is the same, however, this time your acceleration is the wind resistance and rolling resistance of the tyres. You have to produce enough power to overcome these resistances AND the friction of the engine/drive line AND the pumping losses created by the part throttle opening of the engine.

If you increase the opening the air has available to enter into the engine, but DECREASE the air density so that ultimately the same number of air molecules enter the engine, you are reducing the pumping losses and therefore have to produce less power to maintain your speed.

And I'm terribly sorry, but the throttle ONLY controls the air intake. The computer controlls the fuel intake. The throttle is not linked to a valve that lets more fuel in, rather, the throttle IS A VALVE that lets air in and the computer adds fuel in a predetermined ratio to the amount of air going in. This is called the Air to Fuel ratio. You do not talk in fuel to air ratios and tune an engine by dumping x amount of fuel in, them altering something else to allow the right amount of air in.
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Posted: Mon Oct 03, 2005 6:13 pm 
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From the year 12 chem book:
Burning fuel is an extothermic (heat generating) reaction. If the recatants start off colder the reaction will complete quicker and release more energy.

Therefore colder fuel and air = more power and more efficient burn, better fuel economy.

The pumping losses are the same because the air IS still the same density. All you are doing is moving the restricion from the throttle to further up the intake. The same mass of air is still reaching the cylinders, it's just hotter.

 

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Posted: Mon Oct 03, 2005 6:55 pm 
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'unclewoja', what you're discussing is very interesting.

do you have some sort of evidence that supports it ??
are u doing engineering btw, coz the way you explain it sounds like it.

 

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Posted: Mon Oct 03, 2005 7:45 pm 
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ecomony relates to the efficiency of power output per unit volume of fuel.

colder air will result in more oxygen molucules. hence greater a explosion as compared to using hotter air. This allows more power output per unit of fuel.

because of this, the engine achieves a greater efficiency. Therefore, it will require less fuel to push the car foward due to increase efficiency.

like you said, even when travelling at constant speed, that engine is always using fuel to fight against frictional forces. even when travelling at constant speed, the theory that i have just explained still applies.

please feel free to tell me what is wrong with what i have said. because i cant understand y it would be incorrect.

 

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Posted: Mon Oct 03, 2005 9:02 pm 
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partie wrote:
ecomony relates to the efficiency of power output per unit volume of fuel.

colder air will result in more oxygen molucules. hence greater a explosion as compared to using hotter air. This allows more power output per unit of fuel.

because of this, the engine achieves a greater efficiency. Therefore, it will require less fuel to push the car foward due to increase efficiency.

like you said, even when travelling at constant speed, that engine is always using fuel to fight against frictional forces. even when travelling at constant speed, the theory that i have just explained still applies.

please feel free to tell me what is wrong with what i have said. because i cant understand y it would be incorrect.


To travel along @ a constant speed, let's say you need to produce 20kW. If you cool the air down, you produce more power yes. I'm not denying that. But now since you're producing say 22kW, you need to close off the throttle, otherwise you start accelerating and we want to maintain a constant speed, hence increasing pumping losses Let's say you have now increased the pumping losses by 0.5kW. Now you need to produce 20.5kW to do the same speed.

To explain this concept another way, let's talk in AFRs. Basically what people are saying here is that you want to produce the most power possible from the amount of air that's going into the engine. This stands to reason that an AFR of 12.5:1, or best safe power, will be more fuel economical than 16.5:1. Incidentally, the reason that 16.5:1 AFR give you better economy is that the air/fuel charge burns slower (according to spork the slower the fuel burns, less economy you get) and requires more air for the same amount of fuel resulting in a larger throttle opening, hence reducing the pumping losses.
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 Post subject: Re: Intake Air Temp and Pressure vs Economy
Posted: Mon Oct 03, 2005 11:38 pm 
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unclewoja wrote:
Hi all.

I've seen round the forums for a while a few mis-truths when it comes to economy of your vehicle. One I have seen, or mayde it's just a general mis-conception, is that getting cold air will increase your fuel economy.

This is not the case based on the following reasoning.

We all know that if you get colder air into the engine, you increase the engine power by virtue of the fact that the air density is greater and hence, you get more air and fuel into the cylinder.

In trying to get the most power out of your engine, you need to make it as easy for the engine to breathe as possible, and you need to pack as much air into each cylinder as possible.

However, in trying to get the best economy out of your engine, you are solely concerned with making it as easy as possible for the engine to breathe. By contrast to getting the most power out of your engine, in getting the most economy, you should not al all be concerned with how much air you can pack into the cylinder. If you need 15litres/hour of fuel @ 100km/h, you need 220litres/hour of oxygen @ 14.7:1 ARF and since oxygen makes up only 21% of the air, you need 1050litres/hour of air.

This is controlled by the throttle. So, let's say you need 5% throttle to maintain 100km/h based on the above figures. If you decrease the temp of the air, you increase the density which gives you more power which is irrelevent because all this means is that you need to close the throttle to maintain 100km/h.

And therein lies the reason why cold air does not increase fuel economy. One of the biggest variables that impacts on economy is what's knows as pumping losses. The engine's trying to suck x amount of air in, but the throttle is restricting that air flow, so the engine has to work harder to get the amount of air it needs.

By increasing the density of the air and closing the throttle, you are making it harder for the engine to do its work, not easier, thereby making the engine use more fuel.

The answer? Hotter air give you better economy. This decreases the air density meaning you need a wider throttle opening to deliver the right amount of air to the engine. This is only effective however if you can adjust the spark to properly match the intake air temp, as a hotter, less dense air requires more time to burn and hence you need more spark advance.

The other thing that will decrease fuel economy is getting the air from a low pressure area off the car, such as the bumper or the bonnet. Both these areas have air passing over them @ your road speed in nill wind conditions. This has a venturi effect on any hole the air passes over and sucks air out of that hole thereby creating a low pressure area. Getting air from a high pressure area of the car makes it easier for the engine to suck air in and while it means you need a smaller throttle opening thereby increasing pumping losses, this is offset by virtue of the fact that air higher than atmospheric pressure is trying to force its way into the engine.

Incidentally, grabbing air from a low pressure area also decreases your engine's peak power @ speed. This will not be noticed on a dyno, but wind your car up to 150km/h + on a track, or even 100km/h on the freeway, and you engine will be putting out less power than on a dyno.





it's a very well thought out theory.

Though i have problem with it.


i agree that with denser air there is more oxygen and hence a small throttle opening, But using less throttle in the instance will not increase pumping losses. WHY? because with the colder air you have MORE oxygen. and hence for a smaller throttle opening the SAME amount of oxygen is going in. so the engine needs a smaller amount of air to make the same power. Not the same amount. if it was trying to suck the same amout of air through the smaller opening then yes more pumping losses.
the instance that this would happen is if a larger cam was fitted. and we all know what happens to economy then.


if you want to reduce pumping losses focus on getting gas out not in.

 

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 Post subject: Re: Intake Air Temp and Pressure vs Economy
Posted: Tue Oct 04, 2005 10:25 am 
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tickford_6 wrote:
it's a very well thought out theory.

Though i have problem with it.


i agree that with denser air there is more oxygen and hence a small throttle opening, But using less throttle in the instance will not increase pumping losses. WHY? because with the colder air you have MORE oxygen. and hence for a smaller throttle opening the SAME amount of oxygen is going in. so the engine needs a smaller amount of air to make the same power. Not the same amount. if it was trying to suck the same amout of air through the smaller opening then yes more pumping losses.
the instance that this would happen is if a larger cam was fitted. and we all know what happens to economy then.


if you want to reduce pumping losses focus on getting gas out not in.


What about if I talk in CFM? Endothermic reactions and differences in temperature aside, if you need to make 20kW, you neex X amoutn of fuel and y amount of oxygen molecules.

Now, let's say @ 10C, the amount of oxygen required perminute is 20 CFM (cubic feet/min for those of you who don't know) If you decrease the density, the amount of oxygen needed remains the same as you're using the same amount of fuel, but it's taking up more volume. So, now you might need 30CFM if the air temp goes up to 30C.

It takes power to create a vacuum. The more vacuum you need, the more power you need. This can be seen if you switch your engine off going down a hill. If you hold the throttle wide open, you accelerate down the hill due to gravity. If you close the throttle off, you slow down.... depending on how steep the hillis of course. Why? You're needing energy to create the vacuum with the throttle closed and the car is getting this energy from the kinetic energy of the car itself.

So, if you have to suck in a greater volume of air to create the same power, just like if you lean out the engine, you are creating less vacuum and hence need less power.
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Posted: Tue Oct 04, 2005 6:12 pm 
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Interesting, makes alot of sense to me especially with cruise set at 110km/hr for 5 hours on the highway.

I will be doing 1000km in the next week so i will be able to compare the fuel economy of my previous intake system;

1) Tickford snorkel, K&N panel filter, Factory air box.

2) Compared to my new setup of a TFI Pod Filter, 3in TFI steel adaptor with the tickford snorkel directing air to that area. It is not enclosed...

will post the results next week..

Also your right about what your saying about low and high pressure area's because the v8 supercars use the same principal. They get air from up under the grill near the wind shield(high pressure area).

Factory ford intake snorkel is in a low pressure area on top of the radiator support panel and my Pod Filter is in a high pressure area in the space where the air box use to sit. Therefore the engine should suck better and fuel economy will be improved.....

Cheers again...
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Posted: Wed Oct 05, 2005 2:06 pm 
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Ok, I’m no rocket scientist, but wouldn’t it be simpler if we all just installed a turbo or supercharger, then our engines wouldn’t have to suck air at all.. :D
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Posted: Wed Oct 05, 2005 3:38 pm 
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thats why I remove the tickford snorkle. sucking in a mixture of cool/warm air at 60kmh & hot air at the lights.

 

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