Jeep 4.0 Volumetric Efficiency

Anyone know relatively exact number throughout the rev range of the volumetric efficiency? I know its pretty close to the torque curve but does anyone know what that is in % of vol efficiency? I'm looking primarirly at around 1900rpm if that helps. Also, does anyone know of the cross sectional area of a stock Jeep Cherokee?

EPA rates the Jeep 4.0 on the highway with the AW4 at 21pmg, make it 20mpg to make it easier, and at approx 60mph. Could you then say that you use 3 gallons/hour (not factoring in a few things). From then you can say 11.35624 Liters/hour or .18927L of gas per minute. Does that sound right for fuel consumption at 60mph?

Relatively exact? lol

I don't think anyone can give you an exact answer but if I had to guess....75%.

I think the frontal area is about 27 square feet.

Your fuel consumption numbers are right on.

But as always "your mileage may vary."

Basically what I'm trying to do is find out the theoretical mpg you could get given a constant speed (primarily 4th gear locked in cruise at 60mph because 60mph is also a mile per minute and makes for simpler calculations). Anyways, given a constant engine speed, air/fuel ratio (Im guessing 15.5 or so at cruise because I've heard engines run lean at less load for fuel efficieny purposes) I'm trying to find out the miles per gallon. You know the ratio of air to fuel, the mass of air (density is 1.2g/L) and the mass of gas (wiki said 737.22g/L but thats arguable), the engine speed and therefore the amount of intake strokes per minute. Also per minute is how far you are traveling, the "miles portion" of miles per gallon. Anyone try this at all? I got funky results because I know there are variables that I need to find out.

BBeach said:

Anyone know relatively exact number throughout the rev range of the volumetric efficiency? I know its pretty close to the torque curve but does anyone know what that is in % of vol efficiency? I'm looking primarirly at around 1900rpm if that helps. Also, does anyone know of the cross sectional area of a stock Jeep Cherokee?

Click to expand...

Which head? Makes a bit of a difference - there are four basic casting numbers in use, and they all "breathe" a bit differently.

The torque curve should follow the VE curve nearly exactly - in simulation, the AMC242 peaks at about 80%VE (average of all four heads, as I recall,) and around 1900rpm, 72-75% should be a workable number. Sorry I don't have any "real-world" results - just sim - but you can knock a point or two off for real-world parasitic drag and disruption of laminar airflow by the sand-cast surface.

If I knew more about what you were trying to do, I could probably give you more useful results than the above.

As far as figuring fuel consumption for anything specific, EPA ratings are horseradish. Take your volumetric airflow, convert it to mass airflow (I'd have to look up the weight of a cubic foot of air,) and then apply the stoich ratio of 14.7:1. Better, assume you're going to run a shade rich on average - say, 13.7 to 14.0:1. Then, go from there. Convert back to gallons, if you need to.

Say you have a jeep with 4.0, AW4, 3.55 gears, and 235/75R15 tires (right there may be a variable because they may have rated with AX-15 and 215/70R15 for that matter...). At 60mph with the 235's you'll be turning about 700revs/min at the wheel. Therefore its about 1864rpm at the engine. For every 2 turns of the crankshaft, every cylinder will have taken in air (3955cc or about 4.0L). <---Now is it taking in 4.0L or the V.E. x 4.0L? That's my first question. But since the V.E. is just a constant at the exact rpm, I can toss it in later if need be. With the 1864rpm/2 = 932rpm x 3955cc = 3686L/min which is the same as 3686L/mile of Air. To find the mass of air you use the formula M=D/V and find that mass to be 4423.2grams of air per minute assuming 1.2g/L density. Using an air/fuel ratio of 15.5:1, that 4423.2/15.5= 285.37grams of gas per minute. Using wiki's density of gas and the known mass, you find the volume to be 2.5834L gas/minute. But.....thats .6825gallons per minute and therefore 41gallons per hour....which is just a little bit wrong. :huh:

Anyone know where I screwed up and if theres somewhere where V.E. of say 60% is in effect, or that air is only 23% oxygen, or that combustion obviously isn't 100%? And if so, does anyone know how much its off by and why? I'm not sure why I'm doing this but this is what you do when you're bored.

BBeach said:

Say you have a jeep with 4.0, AW4, 3.55 gears, and 235/75R15 tires (right there may be a variable because they may have rated with AX-15 and 215/70R15 for that matter...). At 60mph with the 235's you'll be turning about 700revs/min at the wheel. Therefore its about 1864rpm at the engine. For every 2 turns of the crankshaft, every cylinder will have taken in air (3955cc or about 4.0L). <---Now is it taking in 4.0L or the V.E. x 4.0L? That's my first question. But since the V.E. is just a constant at the exact rpm, I can toss it in later if need be. With the 1864rpm/2 = 932rpm x 3955cc = 3686L/min which is the same as 3686L/mile of Air. To find the mass of air you use the formula M=D/V and find that mass to be 4423.2grams of air per minute assuming 1.2g/L density. Using an air/fuel ratio of 15.5:1, that 4423.2/15.5= 285.37grams of gas per minute. Using wiki's density of gas and the known mass, you find the volume to be 2.5834L gas/minute. But.....thats .6825gallons per minute and therefore 41gallons per hour....which is just a little bit wrong. :huh:

Anyone know where I screwed up and if theres somewhere where V.E. of say 60% is in effect, or that air is only 23% oxygen, or that combustion obviously isn't 100%? And if so, does anyone know how much its off by and why? I'm not sure why I'm doing this but this is what you do when you're bored.

Click to expand...

Yeah, I know what you mean.

As I've highlighted above, you need to multiply the VE (real-world percentage) with the displaced volume/revolution (theoretical quantity) to get the amount of air you're actually moving. So, it's VEx241.5ci, or whatever that works out to in cubic centimetres (~3957.5cc - found my calculator.)

For more detail, I'll probably have to crunch some numbers myself, then "show my work" for it to make any sense. However, I can definitely say you're running a little lean - 15.5:1 isn't something that the computer will usually pass for cruise RPM (ideally, it doesn't go past 15.0:1 under normal operation - and should run just a touch rich, as I'd said, to avoid detonation. The catalytic converter should handle the HC and CO emissions that will result. However, the control computer tries valiantly to run at the "ideal" 14.7:1 ratio.)

Since my wife will be in New York for the week, I might tackle some of this one evening or another, and see what I come up with. If you don't see anything by Friday, would you please PM me a reminder?

I just took 15.5 as a guess, I tried it with a more stoich number and it just requires more fuel which is making my calculations even more off. And yes, I'll try to remind you if I dont forget myself. This is something I've always tried to do but I keep getting stuck on some stupid calculation or am leaving something out. As far as finding out air/fuel, I heard you can wire leads into the O2 sensor and then read it with a multimeter, say from passenger seat and record what's seen. I could leave it on cruise control at 60mph and see what I get, then use that possibly? One of these days I'll invest in a gauge pod and a gauge.

At peak torque, the VE of a stock 4.0 engine is about 77%. At lower rpm the VE is nearer 70%.

These two sites should have all the formulas you need (if you don't already have them)
http://www.epi-eng.com/ET-VolEff.htm
http://www.installuniversity.com/install_university/installu_articles/volumetric_efficiency/ve_computation_9.012000.htm

After reading through that article, there are even more variables including the over inefficieny of the engine. (1/3 30% area). Does anyone have a g-tech pro and had it on the hp setting while on the highway and put it in neutral to see the negative hp produced by the deceleration? I remember doing it before but I forgot the exact numbers, maybe 40ish hp?

No such thing as negative horsepower.

When cruising on the highway at a steady speed the force from the engine cancels out the drag force. This is called 'static equilibrium'.

When you let off the gas the engine isn't applying as much force and the drag force is about the same as before, thus there is an imbalance of forces and you feel an acceleration backwards (negative).

I know that, but im saying it came up with a negative value for hp, prolly because it just plugs and chugs in some formula. Like I asked before, I was wondering if anyone knew what the force required to keep it in that static equilibrium? And like I said before, I put the engine in neutral thus disconnecting it from the rest of the drivetrain therefore the only resistance in the drivetrain is just the gears and such. So basically I was wondering if anyone knew the force of the air resistance amongst other resistances the jeep has to overcome by producing such and such amount of power at the wheels with such and such amount of fuel (therefore finding the theoretical mpg).

Saw this in a link in another thread. This might explain the mass of the air instead of assuming it takes in the 4 liters per 2 revs...

this is a first, an XJ simulation is being built on the naxja board.

http://www.xjjeeps.com/com_xjjeeps/tech_reference.asp?section=2&ID=21
page 5

for a 4wd 2000 model

the drag cooefficient is .51
frontal area is 24.5 sq ft or 2.28 sq m

Another parasitic horsepower loss comes from the rolling resistance of the tires, which increases hysteretically with speed. Not sure where to find a spec for that!

http://www.omninerd.com/2006/07/16/articles/57#endnote_18
That was the site from the link on some other thread. Pretty interesting read, something Im neglecting in the calculations is the load on the engine. Cruising obviously isnt the same as WOT, now I just need to find out what the load on the engine is. Does anyone know of any cheap software and connectors for me to start doing this stuff with my laptop?

Well, you could get a OBD scanner like that guy to read the ECU variables, or possibly splice in vacuum line to MAP and hook up gauge.

I have a MegaSquirt(currently disabled till this weekend) and can measure all sensor variables. I hope to try and calculate fuel real-time to plot, as I know how much fuel im dumping in. It doesn't know about vehicle speed, so I may have try and figure how to add extra input, either that or post process.

I can say the stock ECU will keep O2 readings at stoich unless your really hitting the gas(80% or up) Throttle position value. So if you do hook up a voltmeter to your O2, you can expect to jump very quickly from 0.1 to 0.9 voltages when your not a)warming up(~1 minute) or at WOT(80% throttle or up). the ECU makes sure it stays this way. If you had a wideband you could get an accurate reading, useful for those WOT runs. I would'nt bother with a narrowband sensor.

I left my stock ECU connected, so I can disconnect MegaSquirt control and actually watch and log sensor values while driving under stock control. I can take some MAP readings at various cruising RPMS once my MS is back in. I can say that cruising RPM seems to be 40-50kpa for me, though it can dip lower. I idle at 35kpa(even lower if i richen it up) and cruising can hit this.

I can't say about unplugging. Maybe it will use stop using EGO correction and default to programmed values with no correction values, in this case it would probably run fine given that it had been driven for a while before it was unplugged. However if it was driven in different conditions or loads,rpm's that were never updated with EGO correction, then driveability could be bad. Thats assuming it just ignores O2 after not getting reading. Maybe things would be worse.

CZB83, what are you using an air/fuel ratio for? Forced induction stroker, another vehicle or just for fun?