Hello Everyone,
It’s common sense to realize that accelerating a vehicle uses much more fuel than just driving along maintaining speed. My 2016 Dodge RAM comes with an electronic display that uses a bar graph to illustrate your current miles per gallon.
While cruising along at 60, even 70 mph it’s typical for the bar graph to show fuel economy anywhere between 24 and say 30 mpg. However, accelerate from a stop light and watch the graph plummet to 10 mpg!
So, my question is, I average about 24 mpg with my truck (6 cylinder engine, double cab). If I wanted to maximize fuel efficiency what would be a better strategy:
1: Accelerate quickly (not necessarily flooring it mind you) to your target speed and then using just enough throttle to maintain your speed. This has the downside of using a lot of fuel during acceleration, but the period of heavy gas use would be relatively short, then once up to speed the mpg would go way up.
2: Accelerate slowly to your desired speed. This option would provide much better mpg than a quick acceleration, but would still use more gas than cruising down the highway at speed. The downside to this is that while you’re using less fuel to get up to your desired speed than quick acceleration you are doing so over a much longer time.
So, I’m curious, get to speed quickly and use more fuel over a short period of time or slowly accelerate using less fuel than quickly accelerating but taking much longer to get up to cruising speed. Which style would maximize fuel efficiency long term?
Typically, full throttle at the lowest RPM produces the most fuel-efficient power. You can find a more specific answer by looking up the brake specific fuel consumption plot for your engine. In the linked example, optimal fuel efficiency is around 2200 RPM and just short of full throttle.
It’s easy to achieve that in a manual, where you can just floor the gas and shift up around ~2500 RPM or wherever the optimal point is for your engine. Depending how much torque you have at that range, you might be accelerating at a good rate. Do that in an econobox and you’ll be crawling forward, and you’d better prepare to downshift if there’s a slight hill…
In an automatic, it’s not quite as easy to accelerate without triggering a downshift, since the ECU interprets a floored gas pedal as a request for “MOAR POWER!”. But you can still learn how much gas pedal to give it so that it accelerates briskly without downshifting.
I’ve read that competitors in those ultra-mileage competitions…you know, the tiny, lay-down vehicles that get like 800 mpg…do it by accelerating at full throttle to a certain speed and then coasting. It was explained that the intake tract on the engine is most efficient when the throttle is wide open. Maybe the same principle applies to real-world cars?
This depends very much on the type of car engine. The Prius engine doesn’t do well at high power levels, less time per cylinder stroke means less efficiency. For Teslas I don’t think it matters much, what hurts your efficiency is braking (regeneration isn’t perfect and especially heavy braking where it’s using your brake pads) and high speed driving, because the air resistance is taking energy out of your car constantly.
As others have mentioned the classic strategy is to run wide open throttle for the shortest time and then cruise.
I have played around with this endlessly over the years with our Sorento which has a accurate average/instant mpg reading. One thing I have determined is when cruising at highway speed where there are occasional grades is to switch to manual override and lock it in 5th gear. That prevents a downshift while climbing slight hills.
I recall lots of pop advice media articles, like when gas prices go up a lot, saying you optimize mpg, in stop and go like traffic lights, by accelerating briskly but not full throttle.
The little meters which show your mpg also shows you how much it varies and how rapidly in transient conditions like accelerating, hills etc. It suggests it’s probably complicated to exactly optimize mpg in transient conditions. Even studying an mpg plot is going to show you steady state conditions at each point on the map.
And it would surely depend on the car. In some cars ‘briskly’ means full throttle. In my car full throttle from a standing start means getting thrown fairly hard back in the seat and the traction control light coming on as the wheels begin to spin and car cuts the throttle automatically. Even on dry pavement with the summer tires warmed up, if you literally instantly floor it the light will come on. It seems to me unlikely that’s optimal for mpg.
This is why I would like to see more MPG stat options in cars.
My car tells me the average MPG since I last reset the odometer, but not the instant MPG, nor an average over the last trip or last hour or ten minutes or thirty seconds.
I’d love to have the option of changing the stat display, to give me that information, and it seems as though if that info were available, answering the OP’s question would actually be a pretty straightforward experiment.
But, with just the average MPG per odometer reset, the OP could still answer his question much easier than post #2 indicates. Don’t have to do a full tank one way or the other, just do a single commute one way or the other, and reset the odometer before each run.
Actually, it’s not. The number you actually care about is “Kinetic Energy Gained by Car/Quantity of Fuel”. This would tell you the correct rate of acceleration.
Once at cruising speed, yeah, MPG is the correct number.
Naturally the Prius does most of what you ask for.
This. Well, but with a slight tweak. If you wanted to accelerate to maximum speed with minimum fuel consumed, it would be exactly this. But if you’re comparing strategies for going point to point in a given time with minimal fuel, you’d actually have to account for the fact that spending more time at your highest speed means you can make the trip with a slightly lower highest speed, which makes the MPG better for the constant velocity part of the trip because air resistance is a strong function of speed. If you were, for example, doing all of your driving on city blocks with a stop sign at every corner, this could wind up being important.
Immediate logic says I would want to figure out the coasting distance from max speed to stopped. Then accelerate at absolute minimum to reach that distance from the finishing line. This is only going by wind resistance increasing at a factor of 4.
Fuel economy is largely a function of engine RPM, all else being equal. Therefore, early shifting and an open throttle will give you better mileage. That’s why you want to accelerate as fast as you can without inducing a downshift, because shifting to a higher rpm will kill economy.
Charles Lindberg had a large impact on the war in the Pacific in WWII, as he went over and trained pilots how to run their engines ‘oversquare’, or in a high power, low RPM mode. Pilots where generally taught to prevent ‘oversquare’ conditions because it was thought to be hard on the engines. (oversquare is the condition when your manifold pressure is greater than RPM. So 25" of MAP and 2600 RPM is undersquare, but 25" of MAP and 2400 RPM is ‘oversquare’).
These numbers are only related coincidentally and this isn’t a hard rule that means anything, but it was what pilots were taught to prevent them from damaging the expensive engines. Unfortunately, this leaves you in a low power/high RPM situation, which eats fuel. Lindberg had to optimize fuel economy for his solo trip across the Atlantic, and spent a lot of time working out the best fuel economy setup. He discovered tha the engine would run just fine significantly ‘oversquare’ and that fuel economy went way up in that condition.
The result was that the Pacific pilots dramatically improved their range and could now reach targets much farther away, which accelerated the war.
Many ICE’s will ping and get prematurely worn valves by giving it too much at low RPM’s. Even fuel injected with management computer… eg a 2012 lancer.
And an old mitsishi 6 litre diesel will do it, as will other japanese diesels.
With the petrol non-turbo ones, they just didnt bother with allowing the timing to be so retarded, perhaps because the polution limits would be exceeded. (late ignitition means unburnt fuel ? )
With the turbo diesels, I think they just ensured the turbo provided boost at low RPM ?
Vehicle fuel economy is somewhat complicated. For steady-speed cruising, the energy required per unit distance scales approximately with the square of speed (since it’s mostly dependent on aerodynamic drag). This means that if you cruise at 80 MPH instead of 40 MPH, your engine needs to deliver about four times as much energy per mile.
Astute drivers will of course realize that their fuel economy at 40 MPH is not four times as good as their fuel economy at 80 MPH. And this is because engines, especially gasoline spark-ignited engines, have crummy efficiency at light loads (there is an RPM sensitivity, but load is the more significant factor). This engine efficiency characteristic counteracts the improvement one would expect from driving slower. What one typically finds is that the best fuel economy for most passenger cars comes when cruising at something between 40 and 55 MPH; at speeds above ~55 MPH, the higher drag force means worse fuel economy, and at speeds below ~40 MPH, the engine is operating at such abysmal fuel efficiency that it also results in worse fuel economy.
High load on the engine delivers best fuel economy, but not max load. Take a look at the BSFC plot linked to in post #3: there’s an “island” of best engine fuel effiency, but if you simply flatfoot the accelerator pedal, the ECM enriches the mixture beyond stoichiometric to stave off knock, resulting in a drop in fuel economy. So when you’re accelerating, best economy comes with high load, but not max load.
This meshes nicely with how vehicles are operated during ultra-high fuel economy contests: the vehicle is fitted with a tiny engine that’s operated at high load to accelerate to its target speed, and then the engine is shut off; when the vehicle coasts down to some lower speed, the engine is restarted and again operated at high load to accelerate back up to the upper target speed.
AIUI, this sort of engine operation is an important part of why the Toyota Prius delivers good fuel economy. When the engine is started, it delivers enough power to propel the vehicle, but it also delivers some extra power that’s shunted off to the battery. This means that when the engine is running it’s operating at higher load than it would be without a battery to dump power into, so it’s powering the vehicle more efficiently. Something similar happens for conventional cars when driving in hilly terrain: instead of storing chemical potential energy in a battery, you store gravitational potential energy as you climb hills and release it when you descend them. Your engine operates with enhanced efficiency during the ascents, and (assuming ideal road grades) idles with very low fuel consumption during descents. This goes out the window if the descents are steep enough to require braking, but if no braking is required, you can get surprisingly good fuel economy over hilly terrain.
So if you want good fuel economy in general, and don’t care about courtesy to drivers around you or the comfort of your passengers, I’d recommend the following:
-when accelerating, accelerate briskly, but not flat-footedly.
-set upper and lower speed boundaries around your target speed. Accelerate to your upper speed limit, then take your foot off of the gas and coast; when you reach your lower speed limit, accelerate briskly again to the upper speed limit. Repeat ad nauseam (perhaps literally).
-if you foresee a need to stop or slow, coast for as long as possible beforehand. If you have to hit the brakes, it simply means you should have taken your foot off of the gas sooner.
A lot of good info here, but I’m not convinced the question of load has been adequately addressed. Ultimately, the amount of fuel going into the engine is governed by throttle position, rpm is just a function of this. Consider a car going up a slight incline at, say, 30mph. Which uses more fuel, full throttle in sixth gear or quarter throttle in third gear? It’s the former, right, even though the rpm will be much higher? Admittedly that is a cruising situation rather than an accelerating situation, but if you wanted to accelerate to, say, 40mph, that’s going to be more efficient in third/fourth/fifth than hammeri the throttle in sixth.
For me, the most important thing I learned about increasing fuel economy was that most modern engines use no fuel at all when coasting in gear. It is easy to achieve this in a manual car with good anticipation, and by downshifting sensibly you can often avoid braking altogether. Harder in an automatic but I assume modern auto gearboxes let you do this too.
Correct. Post #3 provided a useful BSFC map, but here’s an even more informative one. Notice the additional smoothly curved lines that descend from the upper left to lower right. These are lines of constant engine power output. If you want to cruise at a steady speed you need a fixed quantity of engine power output, and this determines which of those constant-power lines you’re going to be on. To use something like your example, let’s suppose we need 50 kW of power to cruise up a hill at 30 MPH. Find that constant-power line at the right hand axis, and imagine being in a low gear such that engine RPM is up near 4500; your BSFC is 360. Now imagine upshifting, which moves you left-and-up along that 50-kW curve. Now your engine RPM is around 2300, and you’re stepping pretty hard on the accelerator. Same power output, 50 kW, but now your engine’s BSFC is more like 240, right on that “island” of best efficiency.
The BSFC plot linked to in post #3 says it is for a diesel engine. The plot I just linked to, based on the RPM range, is also for a diesel engine. Notice that the island of best efficiency is fairly round, and that for part-load, the lines of constant efficiency curve upward at high RPM. This is to say that for diesels, efficiency has substantial dependence on RPM. Now compare to this BSFC plot for a petrol engine: the island of best efficiency is more oblong, and for part-load, the lines of constant efficiency are much closer to horizontal. This is to say that for petrol engines, efficiency is more strongly affected by load and less strongly affected by RPM.
You’d have to specify what acceleration you want, thus determining which constant-power curve you’ll be attached to. Then you can look at the BSFC plot and figure out which engine RPM (and therefore which gear) moves you to the best BSFC that’s available along that constant-power line. It’s almost always more efficient to move upward and to the left on the BSFC plot - IOW, high gear + heavy accelerator pedal is almost more efficient than low gear + light accelerator pedal. There are of course other real-world considerations, like engine durability: for example, very low RPM together with very high load for long periods isn’t good for the engine.
Some power is required (wasted, really) in order to keep the engine spinning. This power will either come from fuel, or from the kinetic energy of the vehicle itself. Automatics will do whatever is required to keep the engine spinning at something close to idle speed when your foot is off of the pedal, thus minimizing the amount of power required. If you’re driving a manual and you keep it in gear during decel, engine RPM is somewhere above idle and you’re using more power to do this; the vehicle will decelerate more rapidly. If you downshift, you’re using even more power to maintain that higher engine RPM. If you want the least wasted energy, get that engine RPM as low as possible during decel - for a manual, that means taking it out of gear so you can coast for as long as possible before reaching your planned stopping point (and again, this may conflict with the safety/courtesy considerations that come with operating a vehicle on public roads).
Thanks, but what I still don’t quite understand in that example is that if you have the throttle pedal pressed further down in the second case compared with the first, how is that more fuel efficient? What I tend to find in real-world driving (admittedly in a petrol-powered car with a turbo, which will - I believe - tend to make higher rpm relatively more efficient compared with diesel or non-turbo petrol cars) is that if I am having to use a lot of throttle to climb a hill at low rpm, I can downshift and maintain the same speed with lower throttle but higher rpm. In that case, the latter must be using less fuel, no? Or is that simply another way to state this:
I also have some comments on this:
Absolutely.
Has this always been true, even with old-style torque converter automatics?
Agreed - in the past I have downshifted rather than use the brakes, but even at relatively low rpm (where you won’t risk damaging the engine), the overall wear and tear on the engine is probably slightly more than using the brakes, so I’ll stop doing this forthwith - since if you need to slow down to a stop (and therefore waste energy), utlimately it doesn’t affect fuel efficiency whether you do that via the engine or the brakes. But:
This I can’t entirely agree with - in most situations, coasting in gear (and therefore using zero fuel) is preferable to coasting out of gear (using a little fuel to keep the engine at idle), since quite often you will be coming to a stop anyway. But yes, sometime traffic conditions call for coasting in neutral (which I mentally refer to as my seventh gear).
Because engines have worse efficiency at light load than at heavy load.
The latter is using less fuel per combustion cycle (because of the lighter load), but more combustion cycles per unit time (i.e. higher RPM). The fuel per unit time is the factor of interest, and is the product of both of those parameters, just as power is the product of load and RPM. When you can produce the required power output with either high torque+low RPM, or with low torque+high RPM, it’s almost always more efficient to go with high torque+low RPM.
I see now that I screwed up a link in my previous post. I’ll copy the relevant paragraph here, and install the correct link:
Correct. Post #3 provided a useful BSFC map, but here’s an even more informative one. Notice the additional smoothly curved lines that descend from the upper left to lower right. These are lines of constant engine power output. If you want to cruise at a steady speed you need a fixed quantity of engine power output, and this determines which of those constant-power lines you’re going to be on. To use something like your example, let’s suppose we need 50 kW of power to cruise up a hill at 30 MPH. Find that constant-power line at the right hand axis, and imagine being in a low gear such that engine RPM is up near 4500; your BSFC is 360. Now imagine upshifting, which moves you left-and-up along that 50-kW curve. Now your engine RPM is around 2300, and you’re stepping pretty hard on the accelerator. Same power output, 50 kW, but now your engine’s BSFC is more like 240, right on that “island” of best efficiency.
OK, back to this current post. hopefully that BSFC plot, with its lines of constant power, clarifies what I’m trying to say: for a fixed engine power requirement, you tend to get better efficiency by selecting a gear that makes the engine operate at high load and low/middling RPM.
And just to be sure there’s no confusion, note that when it comes to BSFC, lower numbers are better (as opposed to MPG, for which higher numbers are better).
