 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2007Automobile mileage, speed vs time
I was wondering how speed and engine running time affects gasoline mileage. For instance, driving at 60 mph to go from point A to point B and driving at 25 mph to go the same distance. Would the driving time that it takes to cover the distance at 25 mph burn more gas than going the distance at 60 mph? Usually the faster you drive the more gas is consumed. John > I was wondering how speed and engine running time affects gasoline > mileage. For instance, driving at 60 mph to go from point A to point B > and driving at 25 mph to go the same distance. Would the driving time > that it takes to cover the distance at 25 mph burn more gas than going > the distance at 60 mph? Usually the faster you drive the more gas is > consumed. Yeah, but at 25 mph yer still in second gear ie if you were in a higher gear you would travel a longer distance with the same amount of fuel. > I was wondering how speed and engine running time affects gasoline > mileage. For instance, driving at 60 mph to go from point A to point B [quoted text clipped - 3 lines] > consumed. > John On the surface, you'd think "faster burns more fuel" because wind resistance goes up as the square (or is it the cube? No, I think that's energy used) of speed. But its not that simple. Below 60-80 MPH for most cars, you're still on a fairly flat part of the wind resistance curve so even though the energy used due to wind resistance is changing as the cube of speed, its still small compared to rolling resistance and the internal losses in the drivetrain. What dominates mileage in that case is whether the engine and transmission are running at their optimum points. For example, if you're driving so slowly that the transmission can't shift into overdrive and/or lock its torque convertor, you're wasting more fuel than you would be by driving a bit faster to get in top gear and lock the convertor. Most cars are designed to be efficient in the range of common legal speeds. IOW, in the US where most speed limits are between 40 and 85 mph (and 85 is only posted on a few rural interstates) the mileage is terrible below 20-30 mph, starts climbing as you go up to between 50 and 70 mph, and then falls off very quickly as you go faster than that. > > I was wondering how speed and engine running time affects gasoline > > mileage. For instance, driving at 60 mph to go from point A to point B [quoted text clipped - 24 lines] > terrible below 20-30 mph, starts climbing as you go up to between 50 and > 70 mph, and then falls off very quickly as you go faster than that. Biggest effect on efficiency is what gear you are in; after that, air resistance and friction. So, lowest speed you can maintain in highest gear. All else being equal, i.e. hitting red lights, etc. >>>I was wondering how speed and engine running time affects gasoline >>>mileage. For instance, driving at 60 mph to go from point A to point B [quoted text clipped - 28 lines] > resistance and friction. So, lowest speed you can maintain in highest > gear. All else being equal, i.e. hitting red lights, etc. EXCEPT.... valve timing, spark timing, manifold design, etc. sometimes means that you need to run the engine a little faster than it runs at the lowest speed you can maintain high gear in order for the engine itself to be most efficient. But yeah, somewhere near the "bottom" of your highest gear is generally the most efficient spot to run. > Biggest effect on efficiency is what gear you are in; after that, air > resistance and friction. So, lowest speed you can maintain in highest > gear. All else being equal, i.e. hitting red lights, etc.- Hide quoted text - I have to disagree to some extent. Proper gearing is very important, but a vehicle that is geared too high/low (for the weight, driving conditions, experience, and etc.) can get worse milage than one that is geared correctly for the circumstances. For instance: If you are towing a trailer, being in OD not only hurts the tranny, but also gives poor gas milage. Why you ask, because you have to open the throttle and let more gas into the chambers to give you the power needed to overcome the gearing defect for the conditions. There are many other factors that are just as important than being in high gear. Hank > I was wondering how speed and engine running time affects gasoline > mileage. For instance, driving at 60 mph to go from point A to point B [quoted text clipped - 3 lines] > consumed. > John It is a very complex problem and the optimum speed for each brand and model of car is different. Here are some of the issues. Usually the efficiency of an IC engine is maximized at low rpm but maximum throttle opening. However, car engines have a problem with full throttle and low rpm because of drivability (the mixture does tend to lean out under full throttle. So most engines have max efficiency at about 2/3 to 3/4 throttle, again at low rpm. Aerodynamic drag is the main loss of energy at high speed, but rolling friction is at low speed. The exact crossover speed depends on the streamlining of the car, the tires, bearing design, and vehicle weight. Back in the days of the first oil embargo (mid-70s) there were a number of articles about optimum speeds for various cars. I seem to recollect most were in the low 50s. A low numerical ratio (high gearing) maximizes mpg. In fact, the first "Mustang MPG" model was merely one with a lower numerical rear axle ratio. If you have a stick shift you can maximize fuel economy by short shifting, picking very low rpm shift points. Most FI cars can lug quite well without shaking, which is hard on car. On Feb 13, 10:05 am, "Don Stauffer in Minnesota" <stauf...@usfamily.net> wrote: > > I was wondering how speed and engine running time affects gasoline > > mileage. For instance, driving at 60 mph to go from point A to point B [quoted text clipped - 28 lines] > shifting, picking very low rpm shift points. Most FI cars can lug > quite well without shaking, which is hard on car. Mmm.. yes; thanks to you and Steve and Hustlin' Hank for reminding me that cars use all sorts of power valves and high speed enrichment jets and fancy fuel/air curves in the FI ECU so that wide open throttle might not, in reality, be the most efficient engine speed. Although, having taken apart the carb on my snowblower and found none of those, I now stipulate that the lowest speed which you can keep in high gear is the most efficient speed to drive a snowblower, as long as there's none of that pesky snow to mess up the simple example. > On Feb 13, 10:05 am, "Don Stauffer in Minnesota" > <stauf...@usfamily.net> wrote: [quoted text clipped - 36 lines] > and fancy fuel/air curves in the FI ECU so that wide open throttle > might not, in reality, be the most efficient engine speed. Well, again in THEORY... ;-) Wide-open throttle at a LOW rpm (basically "lugging" the engine) is actually very efficient because you get maximum cylinder-filling, maximum volumetric efficiency, minimum requirement for spark advance, and low RPM means that you're not actually pumping lots of air/fuel thru the engine. The engine may reach its absolute peak thermal efficiency at WOT/high RPM (on the peak of its power curve) but then you don't need all that power to cruise at 70 mph. And if you designed a car that DID use 100% of its engine's available power to cruise at 70, it would be pretty efficient but impossible to drive in the real world because it would take minutes to get to 70 mph, couldn't climb a hill at 70, and couldn't merge into traffic. > > On Feb 13, 10:05 am, "Don Stauffer in Minnesota" > > <stauf...@usfamily.net> wrote: [quoted text clipped - 50 lines] > would take minutes to get to 70 mph, couldn't climb a hill at 70, and > couldn't merge into traffic In a similar vein, that's why aerodynamics of cars is kind of a crock. To really make use of the improvement in gas mileage from the aerodynamic improvement would mean using a smaller engine to take advantage of the lower power required, as you point out, and nobody would buy that car, again as you point out. And it would probably be heavier than the shoebox that was under the aerodynamic improvements, making the situation worse. >>> On Feb 13, 10:05 am, "Don Stauffer in Minnesota" >>> <stauf...@usfamily.net> wrote: Snipped to shorten the article >> Well, again in THEORY... ;-) >> >> Wide-open throttle at a LOW rpm (basically "lugging" the engine) is >> actually very efficient because you get maximum cylinder-filling, Only to the point that pumping work (throttling loss) is reduced as MAP approaches atmospheric pressure. At low-speed the throttle doesn't have to be even close to fully open to achieve this. The other obvious benefit of reducing speed is to reduce mechanical friction (rubbing friction from the bearings, valvetrain and various pumps). >> maximum volumetric efficiency, minimum requirement for spark advance, To reduce throttling losses, one should actually take steps to degrade VolEff. This would make it easier to get to an unthrottled condition at lower torque outputs more in line with cruising conditions. This is an approach used today in many engines with variable cam timing systems. Minimum spark advance is immaterial. There is no absolute spark timing goal. Rather, one wants to set the timing so that the maximum torque is achieved for the particular speed, air-fuel ratio, egr rate and MAP value. You set spark to achieve optimal combustion phasing where 50% of the mass is burned at about 8 deg after TDC. The ignition timing setting to get this doesn't matter. >> and low RPM means that you're not actually pumping lots of air/fuel thru >> the engine. The engine may reach its absolute peak thermal efficiency at >> WOT/high RPM (on the peak of its power curve) but then you don't need Nope! WOT is rarely the most efficient in terms of fuel consumption. Remember, for maximum performance one wants to use up all of the inducted oxygen to burn fuel. Depending on the particular engine's in-cylinder mixing, considerable enrichment may be needed to insure that fuel can find all of the oxygen. For fuel economy at a fixed speed we want to only use enough fuel hold that particular vehicle speed. This means the target torque at the wheels is constant (at the flywheel output too for simplicity's sake). The goal then is to get rid of as much of the engine parasitic losses as is possible - pumping losses, mechanical friction, heat transfer, etc. >> all that power to cruise at 70 mph. And if you designed a car that DID >> use 100% of its engine's available power to cruise at 70, it would be >> pretty efficient but impossible to drive in the real world because it >> would take minutes to get to 70 mph, couldn't climb a hill at 70, and >> couldn't merge into traffic Hypothetically, sure you could do that (and that, in concept, is similar to the vehicles that used to participate in the old Shell, IIRC, fuel economy runs). But, as you point out, that approach would be unacceptable to just about every vehicle buyer. > In a similar vein, that's why aerodynamics of cars is kind of a crock. How do you figure this to be true? Aero drag power increases with the cube of speed. Beyond around 40-45 mph, aero drag begins to dominate the road load requirement. > To really make use of the improvement in gas mileage from the > aerodynamic improvement would mean using a smaller engine to take > advantage of the lower power required, as you point out, and nobody Nothing says you HAVE to downsize the engine. But, even if you don't change the engine, a reduction in aero drag will still help improve F.E, just not as much. If you don't change the mass of the vehicle, then of course the downsized engine will have poorer acceleration. This is why so much time and effort is being put into making engines "behave" at light loads like smaller engines than they really are. (cylinder deactivation, variable valve timing, etc) > would buy that car, again as you point out. And it would probably be > heavier than the shoebox that was under the aerodynamic improvements, > making the situation worse. >Minimum spark advance is immaterial I disagree. Spark advance is used to optimize total power ouput by making cylinder pressure hit its peak at the optimum point in the crank ortation, OR in a completely different oprating regime (at part throttle/low manifold pressure) to act as an efficiency band-aid and start the slower burn that occurs in a low-pressure cylinder early enough so that it can mostly complete by the time the exhaust valve opens. In both of those cases, the price you pay is a big pumping loss in continuing to compress an already-burning mixture. If you are not in an operating regime where you need to eke every horsepower that the engine can produce, you can think differently. If you're trying to cruise 70 mph and use less than 10% of the engine's maximum power capability, a very efficient way to do it is to RAISE cylinder pressure (by opening the throttle and raising manifold pressure) so that the burn goes rapidly, and LOWER rpm so that you're not extracting a ton of power (and conincidentally providing more time for the burn to complete before the exhaust valve opens). In that regime, you can reduce spark advance and eliminate the associated pumping loss from continuing to compress the charge as it is already burning. > Nope! WOT is rarely the most efficient in terms of fuel consumption. 60 years of economy experiments with aircraft and automobile engines disagree with you. But perhaps I should say "near WOT" so that a pedantic assumption of true WOT is not necessary. Back when the Allies were first experimenting with getting figheter escorts to achieve the range needed to accompany the bombers to Germany, the answer was to advance the throttle close to wide open to maximize manifold pressure (including boost on supercharged engines), but set the prop governor to lug the engine down to an absurdly low RPM to maintain a reasonable cruising speed and thus maximize combustion efficiency. The same applies to auto engines, and while they don't quite hit WOT modern cars when in top (O/D) gear basically do the same thing. 1500 RPM at 75 mph is common! Lug the snot out of the engine to maximize cylinder filling, keep the RPM down so that the burn more nearly completes before the exhaust valve opens, and thus extract as much mechanical energy from the heat as possible. Its not quite "WOT," but the throttle is much further open than if you took the same engine, put in in a lower gear, and then adjusted the throttle to cruise at 70. > Remember, for maximum performance one wants to use up all of the > inducted oxygen to burn fuel. Depending on the particular engine's > in-cylinder mixing, considerable enrichment may be needed to insure that > fuel can find all of the oxygen. All true under the caveat you stated of "FOR MAXIMUM PERFORMANCE." I'm not talking about maximum performance, I'm talking about upping the efficiency of a car/engine SYSTEM that still retains the ability to operate in the real world (IOW, has a huge power reserve for passing, accelerating, merging, etc.) We're not talking about trying to get the absolute max efficiency out of just the engine, we're talking about operating a powerful engine at a level on the orderof 10% maximum power but doing so in the most efficient way possible. Slowing everything down from 7000 RPM to 1500 RPM changes everything, including providing plenty of time for the mixture to burn thorougly. Time that DOESN'T exist at high RPM. > For fuel economy at a fixed speed we want to only use enough fuel hold > that particular vehicle speed. This means the target torque at the > wheels is constant (at the flywheel output too for simplicity's sake). > The goal then is to get rid of as much of the engine parasitic losses as > is possible - pumping losses, mechanical friction, heat transfer, etc. Exactly- which is why high manifold pressure/low RPM operation is so efficient. Lower pumping loss through less spark advance, hitting your target torque by operating on the low side of the torque peak, and getting rid of all the parasitic losses (not the least of which is crankcase windage) associated with higher-RPM operation. <snip responses to stuff I didn't say in the first place.> >> Minimum spark advance is immaterial > > I disagree. Spark advance is used to optimize total power ouput by > making cylinder pressure hit its peak at the optimum point in the crank Maybe I wasn't sufficiently clear, but spark timing is merely the knob you turn to get the combustion phasing optimized; which by analyzing over 25 years of in-cylinder pressure data strongly correlates to a 50% mass fraction burn crank angle of ~8 deg after TDC. It also correlates to a peak pressure location of about 14 deg ATDC, although the peak pressure location does have more speed and load dependency. For any operating condition then, spark is adjusted to achieve best phasing wrt CA. If you are trying to say setting spark timing close to TDC is always good for efficiency, then you are wrong. > ortation, OR in a completely different oprating regime (at part > throttle/low manifold pressure) to act as an efficiency band-aid and > start the slower burn that occurs in a low-pressure cylinder early > enough so that it can mostly complete by the time the exhaust valve > opens. In both of those cases, the price you pay is a big pumping loss > in continuing to compress an already-burning mixture. If you are not in The standard pumping loss term applies only to the gas exchange portion of the cycle, exhaust and intake strokes, not late compression stroke. What you are describing is an increase in compression stroke work due to combustion initiation and is most commonly categorized as a time-loss effect. This is a consequence of the fact that combustion is not instantaneous but spans a range of crank angles. And at low loads burn duration tends to get longer due to increased residual fraction and lower in-cylinder gas temperatures. > an operating regime where you need to eke every horsepower that the > engine can produce, you can think differently. If you're trying to [quoted text clipped - 4 lines] > (and conincidentally providing more time for the burn to complete before > the exhaust valve opens). In that regime, you can reduce spark advance No, you don't want to necessarily raise cylinder pressure. But you do want to raise manifold pressure to reduce pumping work. (EGR, Lean burn if possible, and variable valve timing for example) Sure, retarding spark while trying to hold load constant will force you to run higher manifold pressures. But, the cycle inefficiency due to severely retarded spark will far outweigh the reduction in pumping work. Also, the retarded spark can also drive exhaust temperatures to potentially dangerous levels (exhaust valve or catalyst material temperature limits). > and eliminate the associated pumping loss from continuing to compress > the charge as it is already burning. [quoted text clipped - 3 lines] > 60 years of economy experiments with aircraft and automobile engines > disagree with you. But perhaps I should say "near WOT" so that a When I see the term WOT, I take it to mean the full load torque vs speed line of the engine. The condition at which The throttle mashed to the floor. What did you mean by the term? > pedantic assumption of true WOT is not necessary. Back when the Allies > were first experimenting with getting figheter escorts to achieve the [quoted text clipped - 6 lines] > top (O/D) gear basically do the same thing. 1500 RPM at 75 mph is > common! Lug the snot out of the engine to maximize cylinder filling, What you want is high manifold pressures to minimize pumping losses, but still limit the total trapped air mass to achieve the desired engine load while maintaining compatibility with emissions aftertreatment systems (for now, this means running at stoich.). This can be done with high EGR, variable valve timing (limits trapped mass) or cylinder deactivation (makes fewer cylinders run at higher loads). > keep the RPM down so that the burn more nearly completes before the > exhaust valve opens, and thus extract as much mechanical energy from the [quoted text clipped - 18 lines] > of time for the mixture to burn thorougly. Time that DOESN'T exist at > high RPM. If enough time doesn't exist at high speeds, then how do F1 engines make 900 hp at 19000 rpm from on 3.0 liters of displacement? Engine burn rates scale nearly linearly with speed. For modern engines the burn duration in crank degrees only increases slightly with engine speed. This is reflected in the fairly shallow slope of the MBT spark vs. speed curve. >> For fuel economy at a fixed speed we want to only use enough fuel hold >> that particular vehicle speed. This means the target torque at the [quoted text clipped - 5 lines] > efficient. Lower pumping loss through less spark advance, hitting your > target torque by operating on the low side of the torque peak, and Not sure what you mean by this. Do you infer that the light load efficiency is a function of peak torque (or peak torque speed)? It sure does not have to be. > getting rid of all the parasitic losses (not the least of which is > crankcase windage) associated with higher-RPM operation. I think we already agreed that for fuel efficiency, high speed operation is out of the picture. If you are bringing the ultimate WOT perf. back in, then sure windage should be considered (and usually is in modern automotive engines). |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.