City driving econ: Faster accel in lower gears vs. Slower accel in higher gears?

[Ankleface]

Hi there, I have a very small and light car with a tiny non-turbo 3-cyl DOHC indirect injection engine using throttle-by-wire and a 5-speed manual trans. I mainly use it for a daily short trip (not possible to combine together with other trips) in a small city with lots of traffic lights that are really poorly synchronized, on my route I often end up hitting all 5 lights on red over 75% percent of the time. There are also quite a few moderate hills, and unfortunately a steep climb to leave my neighborhood while the engine is still not warmed up. I use the air conditioning very sparingly, but it doesn't seem to affect the fuel consumption much either way.

I'm fairly experienced with the principles of efficient driving in small cars, and I used to easily get 10 MPG over the window sticker fuel economy on a slightly larger 4-cyl 5-speed manual that I used to drive in a different rural+city mixed driving route. But I'm extremely unimpressed with the consumption of my current car in my current environment, which is getting 10 - 12 MPG *less* than the manufacturer's rating. I run the highest practical tire pressure, I meticulously maintain the vehicle, and I'm pretty sure that it's not a mechanical problem because the consumption on longer trips meets or exceeds the window sticker rating.

The first culprit seems to be accelerating up the hill out of my neighborhood in 1st or 2nd gear before the engine is warmed up. At that stage I can feel that the engine is still running rich and hesitates more than it does when it's up to operating temperature, and by the time I reach the top of the hill the average fuel economy drops by ~0.5 MPG. I wonder if I should #1) let the engine idle a bit longer to warm up first in my driveway (the owner's manual recommends just 15 seconds of warmup) and/or #2) Go up the initial hill in 1st gear all the way or #3) Try to go up in 2nd as soon as as possible with my foot to the floor. (It's a moderate climate, so "cold" engine is just a relative term.)

And then when accelerating from stop lights, I'm trying to decide which is better: #1) Faster acceleration with higher RPMs for a shorter period of time to get out of the lower gears more quickly and possibly even skip shifting to reach 4th or 5th as soon as possible, or #2) Moderate acceleration around 2000 RPM with higher load in 2nd - 3rd usually foot-to-the-floor to reach cruising speed at 4th or 5th. I'm pretty sure that the #3) option of leisurely acceleration with lower load through all the gears until reaching 4th or 5th would use the most fuel. I'm concerned about the effects of "lugging" the engine, but it's often unavoidable because it has very low torque, and I never let it shake or struggle either. And I know that in theory if the throttle is wide open and dumping in lots of fuel that isn't getting fully combusted then it would be inefficient, but I don't think that a modern throttle-by-wire fuel injected system would do that even if I am foot-to-the-floor.

Thanks in advance for any suggestions and insight.

[freebeard]

First culprit: Have you considered a block heater?

Accelerating from stop lights: I'm for clearing the intersection expeditiously, at the least. My current ride has an 'Ecolight' that prompts short-shifting through the gears, but I ignore it in higher gears on uphill gradients. It can't look ahead.

Are the traffic lights on demand so it depends on cross-traffic? If you can see the (timed) lights a few intersections ahead, my son observes whether there is a wave going away or toward him.

If you roll up on a red light slowly (ecomodder.com › blog › hypermiling-101-driving-without-brakes, often times you can avoid a complete stop.

HTH

[Ankleface]

Hi there, thanks a lot for the reply.

Originally Posted by freebeard

First culprit: Have you considered a block heater?

Hmm, no because it's a very mild climate, bordering on hot. When I say that the engine is "cold" I just mean relative to its normal operating temperature. But even so the engine definitely is less responsive for the first few minutes of driving, basically it feels like it's running a "choked" air/fuel mixture. So it's a double hit to have to accelerate up the initial steep hill while it's dumping more fuel in at the same time. I'm not sure if I can minimize that initial hit by maybe wasting a bit of fuel at zero load in idle for a few minutes and then accelerating all the way up the hill in 1st gear at lower load than if I were to shift into 2nd.

Are the traffic lights on demand so it depends on cross-traffic?

Nope, no sensors, and just random timing that makes it so if you hit one red light you hit them all.

If you roll up on a red light slowly, often times you can avoid a complete stop.

Yep, I religiously look ahead and roll up to red lights in gear, so zero fuel consumption according to the instantaneous display. Probably my average economy would be considerably worse still if it weren't for that tactic.

Thanks for the helpful and friendly response, I appreciate it.

[freebeard]

Hmm, no because it's a very mild climate, bordering on hot.

Maybe that just means a small block heater? in the last Century I used to put an incandescent light on the ground under the engine (in freezing weather). It would be a simple experiment to say if it's worth pursuing.

[cRiPpLe_rOoStEr]

What cars are you comparing? Could you be more specific about the models?

Originally Posted by Ankleface

I have a very small and light car with a tiny non-turbo 3-cyl DOHC indirect injection engine using throttle-by-wire and a 5-speed manual trans.

Some folks tend to believe displacement alone plays a major role in fuel savings regardless of other factors such as gear ratio, ignoring how a shorter gearing and a much higher revving on cruise speed can render a small car fairly thirsty on highway.

I used to easily get 10 MPG over the window sticker fuel economy on a slightly larger 4-cyl 5-speed manual that I used to drive in a different rural+city mixed driving route.

How larger was the displacement of the 4-cyl engine?

[Ankleface]

Hi there cRiPpLe_rOoStEr, thanks a lot for your reply as well.

The previous car I mentioned was a Toyota Corolla with a 1.6L 4-cyl that I owned 20 years ago. And now I have a Hyundai i10 with 1.0L 3-cyl. I guess that was sort of an apples-to-oranges comparison because the two vehicles are not really very comparable. And the area where I used to drive the Toyota was a more spread-out city with less congestion and higher average speeds. I mainly mentioned that example because I've always been pretty conscientious about efficient driving, and my method of "accelerate briskly to the highest possible gear" worked effortlessly in the Corolla to get much better city consumption than the official rating.

You mention a good point about the displacement not being the only factor that affects fuel economy, I've definitely seen that too in other cars. But that's one of the reasons why I'm a bit confused about the high city consumption that I'm seeing now, because in my normal routes around town I never get anywhere close to the 100 KPH speeds where the final drive ratio would start to make a big difference. The Hyundai i10 was built primarily as a city car, and the gear ratios feel about right for that purpose, whereas on motorways it revs well over 3000 RPM at 100 KPH or above. Despite that it still has good consumption at those higher speeds even though it definitely feels out of sorts, whereas in the city environment where it was mainly designed to perform it's relatively thirsty. Going back to the example of the Corolla, that one didn't have a tachometer but it also sounded very revvy on the motorway. I assume that its sedan/saloon design would have been more aerodynamic at higher speeds compared to the Hyundai's relatively tall hatchback design, but at very low city speeds generally under 50 KPH aerodynamics shouldn't play a major role.

So again I realize that the two cars aren't really comparable, and the area where I drive most of the time now is different. I'm not sure about the power-to-weight ratio, based on acceleration I think it was probably better in the Corolla with its ~100 HP and ~135 Nm of torque, while the i10 only has like 65 HP and less than 100 Nm. The Corolla was more of a general purpose car with a fairly normally displacement, whereas the Hyundai i10 has one of the smallest engines currently available in a car. So that's why I feel like there must be some kind of optimization needed in my driving style now for the Hyundai. I'm not a habitual speeder, but I also don't like to go slower than everyone else just to save fuel. I've always accelerated fairly briskly to get out of the inefficient gears, and I anticipate red lights.

Sorry for the wall of text. ;-) Appreciate the replies.

[IsaacCarlson]

A smaller engine does not get better economy by default.

If you were simply rolling around at 25 mph all day on flat ground, then it might. BUT, that's not how driving works. We have to start from "cold", we have stop signs/lights, hills, corners, etc... a smaller displacement engine has to work much harder (more air/fuel) to do the same work as a larger engine, except at very low speeds and loads where internal friction makes a difference. (And in a chrysler, where it doesn't matter how you drive, the mileage is still bad)

We had a 1996 olds with 3.8L and it got 31/32 mpg all the time. Hand built motor with gobs of power. We never wanted more from that car.

We had a 2002 sunfire with a 2.2L and we always wanted more power. It revved too high and was underpowered. The economy was around 28 mpg on country roads, and would drop when in town or at highway speeds. (I have the engine/trans from the olds and plan on putting it in the sunfire.)

My truck has a 4.9L inline 6 and has gobs of torque and enough hp to make me happy. It gets around 25-26 mpg avg between empty and loaded. (Loaded meaning 4-5,000 lbs of firewood) It would easily zip right up to 120+ mph with the old 3.55 gears and not even care. It could get 28-32 mpg with the 3.55.

A (well designed) bigger engine will get a vehicle up to speed without needing higher throttle opening. Take a look at BSFC maps of different engines and compare the fuel to output ratios.

A bigger engine will use much taller gearing than a smaller one because the smaller one needs all the help it can get. Shorter gearing combined with an underpowered engine will use a lot more fuel at higher rpm and load than a bigger engine that is just idling under the same load.

I can drive my truck without going over 1,000/1,500/2,000rpm. Pick any of those numbers, because the only thing they limit is the top speed. If an engine can't effectively power the vehicle at those rpm, it's too small.

Torque is what gets things moving, and can be directly measured. It's what you feel when you take off. Hp is a number we have to calculate and is rpm dependent. If you need high rpm, your economy is not going to be very good.

A chainsaw has very low torque but high hp, with the hp being ~2.5x the torque. They have to rev way up to keep from stalling under load, and the economy is not great.
1-3 ft lbs of tq and 3-8 hp

A large diesel truck has a ton of torque and not a lot of hp, with hp being ~1/4 of the torque. The engine runs just fine at very low rpm's and will not stall. The economy is very good considering how heavy the trucks are.
2,000 ft lbs of tq and 4-600 hp.

My truck has ~260 ft lbs of torque and 150 hp at relatively low rpm. Great fuel economy.

Compare that to a 3 cylinder engine with 70 ft lb and 70 hp. It's not much torque and it's at 3,500 rpm. The hp is at 6200 rpm, which means the car is going to be unbearably slow with ok economy or a little quicker with poor economy.

The problem here is bad business and economics. People like a cheap car that looks flashy, but they gripe when it breaks. Manufacturers don't want to spend money on important things, but will spend it all on flashy things. It is cheaper to build a high rpm 1L engine than a low rpm 2L engine. The common thought is that less weight means more mpg, and that smaller engines get better economy. It's a "forked tongue" statement. It's sort of true, but only in certain situations.

A lighter car only gets better economy when taking off or climbing hills. A smaller engine only gets better economy under much lighter loads. A small reduction in vehicle weight does not warrant an even more underpowered engine that has to work in a higher rpm range.

Let's look at the sunfire/3800 combo for a minute. The car would have about twice the power and be ~800 lbs lighter. My estimates are around 40 mpg. The engine is heavier, but it would be in the back seat, which is a pain to get into anyway, so it would "still" be a 2 seater and the balance would be much better.

I think you should have kept the corolla.

[Ecky]

My experience has been that, with non direct injection + OTTO cycle engines I've driven, short shifting and staying at higher load has generally given the best results. For example, in my Insight, shifting so I'd land at 1000-1250rpm in the next gear generally delivered the best fuel economy, whether it was with the 1.0L or the 2.4L I later put in.

With Atkinson cycle + direct injection engines, staying a bit away from 100% load seems to make a difference. At low RPM and high load, it drops more deeply into pulled ignition timing and fuel enrichment, making mid-RPM (2250-3250) mid-load more economical. There are instances where it's even more economical to cruise in a lower gear.

It might be enlightening to see what Hyundai's fuel and spark maps are doing.

[Piwoslaw]

Not going into your question of how to accelerate, I'll agree with freebeard about installing a coolant heater, assuming you have access to a place to plug it in. No excuses about warm climates - many Ecomodders plug theirs in year-round.
If not an inline coolant heater, then maybe a heating pad that you can stick onto the engine block and/or oilpan?

As these are short trips in a mild climate, then maybe the best way to save fuel would be a bicycle or e-bike?

BTW, have you seen this thread?
SHORT TRIPS? FAST WARM UPS? MPG?

[Ankleface]

So I just did some testing on a 3km loop, and the results were very interesting. About 2/3 of the route is 50km/h city streets with tons of turns and speed bumps and stop signs (no traffic lights to reduce testing variability), and the last 1/3 is 80km/h highway with a hill climb and a flat-ish section. I repeated each method at least twice, and I kept the A/C running the whole time to simulate a worst case scenario. Most of the tests weren't perfect, but the results were pretty consistent with a clear winner. Here are the averages:

Method #1: Moderate throttle and obey the upshift indicator to reach the speed limit: 7.5 L/100km

Method #2: Get out of 1st ASAP, foot-to-the-floor in 2nd until reaching 50km/h, and to reach 80km/h also foot-to-the-floor in 3rd, then use the highest gear possible to maintain speed without noticeable lugging: 6.8 L/100km

Method #3: Get out of 1st ASAP, then foot-to-the-floor and upshift as soon as possible around 2000 RPM until reaching the speed limit, then use the highest gear possible to maintain speed without noticeable lugging: 7.3 L/100km

I'd be interested in hearing your theories as to why things worked out that way. The winning method #2 sounds more or less like the "pulse and glide" technique than many hypermilers recommend. I also forgot to mention that the engine has variable valve timing, so maybe it performs relatively more efficiently in that mode at higher RPMs? I guess that a super small engine would obviously be super efficient puttering around at a slow-ish steady speed, but it appears that it has to inject quite a bit of fuel to rev up to higher RPMs and also to sustain high load. So it looks like spending the shortest time possible in the higher revs with lower load and then taking advantage of its sweet spot cruising at low RPM and low load and then coasting to a stop is the most efficient method.