Random question on highway MPG
04-05-2016, 12:37 PM
#1
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Random question on highway MPG
I clearly didn't purchase/modify my JKU for fuel efficiency; however, I found myself wondering something today that I'd like to see if anyone can answer with at least an educated guess.
Given my lift, heavy additions of equipment, 37s, etc. it can be a challenge to maintain highway speeds in excess of 70-75mph. I find I must leave it in overdrive-off mode and tool along in third gear @ 3-4k rpms (with only maybe 10-15% throttle needed), else if it shifts into 4th the rpm's will drop under 3k, but it might take 30-40% throttle to maintain that speed. If the wind resistance is such that a heavy throttle in 4th gear only maintains speed without increasing it, which gear is more efficient?
Ignoring completely the fact that lugging an engine is detrimental, if you purely consider the variable of light throttle/high rpms vs. heavy throttle/low rpms, which is more efficient at maintaining a set vehicle speed??
All things being equal, I know higher RPMs clearly burn more fuel than lower RPMs, but how does the throttle position play a factor? The heavier throttle at low RPM's would open the throttle plate for more air to enter, but since the RPMs don't increase, what effect does it have? Is the JKU dumping more fuel in to try and accelerate in response to the open throttle plate (and consequently running rich/wasting fuel), or is it possibly running lean due to too much air to fuel ratio? Or is throttle position meaningless if RPMs remain the same and 2k RPMs always burns the same amount of fuel regardless of the throttle opening (again, given the parameter that the wind resistance prevents the RPM's from increasing in response to that throttle input).
Hope the above makes sense. In my experience I haven't seen much real world difference in MPG when using either method, I'm just really curious what people that know more than me have to say about it....
Given my lift, heavy additions of equipment, 37s, etc. it can be a challenge to maintain highway speeds in excess of 70-75mph. I find I must leave it in overdrive-off mode and tool along in third gear @ 3-4k rpms (with only maybe 10-15% throttle needed), else if it shifts into 4th the rpm's will drop under 3k, but it might take 30-40% throttle to maintain that speed. If the wind resistance is such that a heavy throttle in 4th gear only maintains speed without increasing it, which gear is more efficient?
Ignoring completely the fact that lugging an engine is detrimental, if you purely consider the variable of light throttle/high rpms vs. heavy throttle/low rpms, which is more efficient at maintaining a set vehicle speed??
All things being equal, I know higher RPMs clearly burn more fuel than lower RPMs, but how does the throttle position play a factor? The heavier throttle at low RPM's would open the throttle plate for more air to enter, but since the RPMs don't increase, what effect does it have? Is the JKU dumping more fuel in to try and accelerate in response to the open throttle plate (and consequently running rich/wasting fuel), or is it possibly running lean due to too much air to fuel ratio? Or is throttle position meaningless if RPMs remain the same and 2k RPMs always burns the same amount of fuel regardless of the throttle opening (again, given the parameter that the wind resistance prevents the RPM's from increasing in response to that throttle input).
Hope the above makes sense. In my experience I haven't seen much real world difference in MPG when using either method, I'm just really curious what people that know more than me have to say about it....
04-05-2016, 06:15 PM
#2
JK Junkie
I clearly didn't purchase/modify my JKU for fuel efficiency; however, I found myself wondering something today that I'd like to see if anyone can answer with at least an educated guess.
Given my lift, heavy additions of equipment, 37s, etc. it can be a challenge to maintain highway speeds in excess of 70-75mph. I find I must leave it in overdrive-off mode and tool along in third gear @ 3-4k rpms (with only maybe 10-15% throttle needed), else if it shifts into 4th the rpm's will drop under 3k, but it might take 30-40% throttle to maintain that speed. If the wind resistance is such that a heavy throttle in 4th gear only maintains speed without increasing it, which gear is more efficient?
Ignoring completely the fact that lugging an engine is detrimental, if you purely consider the variable of light throttle/high rpms vs. heavy throttle/low rpms, which is more efficient at maintaining a set vehicle speed??
All things being equal, I know higher RPMs clearly burn more fuel than lower RPMs, but how does the throttle position play a factor? The heavier throttle at low RPM's would open the throttle plate for more air to enter, but since the RPMs don't increase, what effect does it have? Is the JKU dumping more fuel in to try and accelerate in response to the open throttle plate (and consequently running rich/wasting fuel), or is it possibly running lean due to too much air to fuel ratio? Or is throttle position meaningless if RPMs remain the same and 2k RPMs always burns the same amount of fuel regardless of the throttle opening (again, given the parameter that the wind resistance prevents the RPM's from increasing in response to that throttle input).
Hope the above makes sense. In my experience I haven't seen much real world difference in MPG when using either method, I'm just really curious what people that know more than me have to say about it....
Given my lift, heavy additions of equipment, 37s, etc. it can be a challenge to maintain highway speeds in excess of 70-75mph. I find I must leave it in overdrive-off mode and tool along in third gear @ 3-4k rpms (with only maybe 10-15% throttle needed), else if it shifts into 4th the rpm's will drop under 3k, but it might take 30-40% throttle to maintain that speed. If the wind resistance is such that a heavy throttle in 4th gear only maintains speed without increasing it, which gear is more efficient?
Ignoring completely the fact that lugging an engine is detrimental, if you purely consider the variable of light throttle/high rpms vs. heavy throttle/low rpms, which is more efficient at maintaining a set vehicle speed??
All things being equal, I know higher RPMs clearly burn more fuel than lower RPMs, but how does the throttle position play a factor? The heavier throttle at low RPM's would open the throttle plate for more air to enter, but since the RPMs don't increase, what effect does it have? Is the JKU dumping more fuel in to try and accelerate in response to the open throttle plate (and consequently running rich/wasting fuel), or is it possibly running lean due to too much air to fuel ratio? Or is throttle position meaningless if RPMs remain the same and 2k RPMs always burns the same amount of fuel regardless of the throttle opening (again, given the parameter that the wind resistance prevents the RPM's from increasing in response to that throttle input).
Hope the above makes sense. In my experience I haven't seen much real world difference in MPG when using either method, I'm just really curious what people that know more than me have to say about it....
The throttle plate is controlled by the PCM, but even if it wasn't, air flow (and therefore fuel flow) is not proportional to throttle opening alone! Air flow is also based on the differential pressure across the throttle plate. So let's do a thought experiment with the example above, but we'll imagine that the efficiency is exactly the same when cruising on flat land at 70 MPH in either third or sixth gear. Since efficiency is the same, both fuel and air flow are the same, but the RPM's are obviously different. In third gear the RPM's are over 5K and the throttle plate is much more closed/restricted and the MAP (manifold absolute pressure, the pressure downstream of the throttle) is fairly low. On the other hand, in sixth gear the throttle is opened further and the MAP is higher. The airflow, fuel flow, and HP are the same in both cases for this same-efficiency thought experiment -- If the differential pressure is more, the throttle opening must be less to get the same air flow.
Anyway, there's lots of things that figure into efficiency. One item is air pumping losses; when there is a low MAP it takes extra power from the crankshaft to pull the piston down on the intake stroke. The low pressure is literally trying to suck the piston the opposite direction, and the lower the MAP (higher vacuum) the more pumping losses which decrease efficiency. So there is a potential advantage to lower RPM's because the MAP will be higher (and pumping losses will be less). Another example is when the A/F (air/fuel) ratio is changed to a richer value at some point to make more power without over-heating engine parts, which is going to drop efficiency dramatically.
Last edited by Mr.T; 04-05-2016 at 06:19 PM.
04-06-2016, 06:23 AM
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Here's a few thoughts on your question. For a given HP (horsepower) demand there's a most efficient RPM to make that HP at, and every engine is different. For example, mine has a manual trans and it runs a little over 5K rpm at 70 MPH in third gear -- This is great for passing on a hill and making full power, but for cruising on the flat at 70 MPH it's very inefficient compared to sixth gear at about half that RPM.
The throttle plate is controlled by the PCM, but even if it wasn't, air flow (and therefore fuel flow) is not proportional to throttle opening alone! Air flow is also based on the differential pressure across the throttle plate. So let's do a thought experiment with the example above, but we'll imagine that the efficiency is exactly the same when cruising on flat land at 70 MPH in either third or sixth gear. Since efficiency is the same, both fuel and air flow are the same, but the RPM's are obviously different. In third gear the RPM's are over 5K and the throttle plate is much more closed/restricted and the MAP (manifold absolute pressure, the pressure downstream of the throttle) is fairly low. On the other hand, in sixth gear the throttle is opened further and the MAP is higher. The airflow, fuel flow, and HP are the same in both cases for this same-efficiency thought experiment -- If the differential pressure is more, the throttle opening must be less to get the same air flow.
Anyway, there's lots of things that figure into efficiency. One item is air pumping losses; when there is a low MAP it takes extra power from the crankshaft to pull the piston down on the intake stroke. The low pressure is literally trying to suck the piston the opposite direction, and the lower the MAP (higher vacuum) the more pumping losses which decrease efficiency. So there is a potential advantage to lower RPM's because the MAP will be higher (and pumping losses will be less). Another example is when the A/F (air/fuel) ratio is changed to a richer value at some point to make more power without over-heating engine parts, which is going to drop efficiency dramatically.
The throttle plate is controlled by the PCM, but even if it wasn't, air flow (and therefore fuel flow) is not proportional to throttle opening alone! Air flow is also based on the differential pressure across the throttle plate. So let's do a thought experiment with the example above, but we'll imagine that the efficiency is exactly the same when cruising on flat land at 70 MPH in either third or sixth gear. Since efficiency is the same, both fuel and air flow are the same, but the RPM's are obviously different. In third gear the RPM's are over 5K and the throttle plate is much more closed/restricted and the MAP (manifold absolute pressure, the pressure downstream of the throttle) is fairly low. On the other hand, in sixth gear the throttle is opened further and the MAP is higher. The airflow, fuel flow, and HP are the same in both cases for this same-efficiency thought experiment -- If the differential pressure is more, the throttle opening must be less to get the same air flow.
Anyway, there's lots of things that figure into efficiency. One item is air pumping losses; when there is a low MAP it takes extra power from the crankshaft to pull the piston down on the intake stroke. The low pressure is literally trying to suck the piston the opposite direction, and the lower the MAP (higher vacuum) the more pumping losses which decrease efficiency. So there is a potential advantage to lower RPM's because the MAP will be higher (and pumping losses will be less). Another example is when the A/F (air/fuel) ratio is changed to a richer value at some point to make more power without over-heating engine parts, which is going to drop efficiency dramatically.
If I am interpreting you correctly, in your thought experiment the low rpm/heavy throttle combination would be more efficient at first due to less pumping loss; however, if the load/duration/whatever pushed the PCM to modify the A/F richer than that efficiency might drop so much that the high rpm/light throttle combination might become better - correct?
04-06-2016, 06:25 AM
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04-06-2016, 06:44 AM
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04-06-2016, 07:01 AM
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04-06-2016, 09:47 AM
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JK Junkie
These are exactly the thoughts/questions that are running through my head, although I'm not sure this got me any closer to an answer...lol
If I am interpreting you correctly, in your thought experiment the low rpm/heavy throttle combination would be more efficient at first due to less pumping loss; however, if the load/duration/whatever pushed the PCM to modify the A/F richer than that efficiency might drop so much that the high rpm/light throttle combination might become better - correct?
If I am interpreting you correctly, in your thought experiment the low rpm/heavy throttle combination would be more efficient at first due to less pumping loss; however, if the load/duration/whatever pushed the PCM to modify the A/F richer than that efficiency might drop so much that the high rpm/light throttle combination might become better - correct?
I put an Aeroforce gauge in mine that connects to the OBD port, and A/F ratio is one of the things it can monitor. The A/F ratio does not richen until the engine is at a power level quite a bit above highway cruise, so that's not a factor until climbing, passing, or having more fun than the law allows.
The best way to increase MPG is to drive slow, especially on hills. The power required to go faster isn't linear, and fast on hills likely will richen the mixture and drop efficiency further. Having said that, occasional full throttle and 5K RPM's keeps the combustion chamber's nice and clean.