Why, exactly, are large-displacement engines more efficient?

General Discussion
21

No one has yet to discuss flame front travel speed nor friction.

First off, friction is a big factor in efficiency. Big bores mean a large circumference of the piston ring surface. The rings still need to rub to seal effectively. Yeah, oil is there but it can’t act like an oil fed bearing because then the rings won’t seal. There is no getting away from that. And a 38 inch piston had a bunch of ring rubbing area! Flat tappet cams also have this issue. Oil fed bearing areas have less effect on friction since they ride on the pressure fed oil film.

Second is flame front travel speed. The bigger the piston, the more time it takes to burn the charge and the lower the efficiency. That alone may prove that a large engine has lower efficiency than a smaller one. The Otto cycle used in 4 stroke engines depends on the pressure pushing the piston down. The longer that pressure takes to build, the less work it can create.

Forget the specific power output of sport bike engines, they turn 12,000 rpm and don’t make good power until 5000 rpm. Turn an auto engine at 13,000 and you will see 200 hp per liter. Honda’s S2000 engine redlined at 9000 and made 240 hp from 2000cc.

22

I would consider efficiency to be the power output relative to the the fuel consumed and a few searches have given me no source for that specific information.

23

Disagree that small displacement is the way to go. It’s great if you only need peak hp sporadically–I grant you that. Low RPM cruise, downshift to “dremel mode” when you need power.


But in applications where you actually cruise at 75% of max rated power–planes, trains, and trucks–ALL of 'em favor low-RPM tractor motors. You COULD make a prime miver that was just so many SBCs stacked end-to-end: they don’t.


You got the “square/cube rule,” for one. If you double all dimensions, you get 8x displacement, but only 4x surface area. That means less heat transfer and less ring friction per cu in.
The most thermally-efficient engines made are LOW-revving 2-stroke diesels…like 150RPM or so.

24

Larger displacement engines generally have more torque at lower rpm which is essential. Generally speaking, the faster an engine has to turn over to do the job to develope necessary torque, the more gas it uses. The alternative to making small engines more efficient, is to make cars light enough not to require much torque to accelerate, use an electric motor to accelerate at low speeds, variable valve timing and turbos. It’s a balancing act and saying larger displacement engines are more economical neglects it’s application, the work it was designed to do and the vehicle it resides within.

And then, there is the transmission. More gears used efficiently can do wonders for mileage whether the motor is large or small. So, a more capable transmission can mitigate generalizations like this.

25

A larger engine may have a smaller surface area to displacement ratio, but it may be more efficient only on the test stand. When you have a bigger engine, everything else attached to the engine has to be bigger. The radiator has to be bigger, more metal is required to mount the engine, the car punches a bigger hole thru air, the transmission has to be strengthened, the suspension needs to support extra weight, bigger brakes are required to bring the extra weight to stop, and bigger wheel are required to clear the big brakes… The entire system becomes less efficient

26

Relative efficiency of an engine has no relationship to radiator size or bigger brakes @chunkyazian. Relative efficiency of one vehicle to another would be affected by those components but not the engine itself.

27

I found this wiki page and it seems to rate engine types according to fuel consumption/power produced, i.e. Brake Specific Fuel Consumption

And a huge diesel is the most efficient. I was amazed at the effiency of diesel aircraft engines and compound gasoline aicraft engines which use turbo chargers to add power directly to the crankshaft.

28

Here’s a good explanation of engine efficiency. It points out that compression ratio is an important factor, which is why diesels are more efficient than gas engines. The higher compression of a diesel allows the combustion to take place at a higher temperature than a gasoline engine, with resulting higher efficiency.

The basic idea is to maximize the temperature T(hot) at which fuel is introduced into the combustion chamber, and to minimize the temperature T(cold) of the exhaust gases. Optimizing these two variables maximizes the amount of work extracted from the fuel and minimizes the amount of energy wasted as exhaust heat.

29

Another advantage of smaller engines is lower standby (idle) fuel consumption. From what I gather from browsing on the internet, in urban driving, standby fuel consumption accounts for about 17% of the fuel use in a typical car.
The 1500 cc four in my Yaris is said to consume about .18 gallons per hour idling. Large V-8 engines can easily burn over 1/2 gallons/hour idling.
One reason little 125cc scooters get such good city mileage is that they only consume mere ounces of fuel per hour while waiting for the red to turn green.

30

@B.L.E.
You have a Yaris ? We are about to rent one all next week while my wife’s car gets repaired after “maiming a fox”. I am not looking forward to it. ;() but they are great on gas !

31

When I test drove a Toyota tercel for my unlicensed college roommate, I was thoroughly enjoyed it. Direct manual steering, no play manual shifter, no mushy Toyota brakes, ice cold ac, distributorless ignition, non interference engine. Even though the engine was underpower, it goads you into driving it hard. And then it all went down hill when Toyota tried to convince us the echo was the better car. It’s so numb that you can’t trust it enough to push it

32

@chunkyazian‌

Sounds exactly like my 1994 Tercel

Manual steering
manual windows and locks
4 speed stick
ice cold ac . . . never had a leak
brakes work great . . . new rotors and pads, rear shoes still have plenty of meat

Over the years, I’ve upgraded it on the cheap. I ditched the vinyl seats, in favor of factory cloth seats from a junkyard donor. I also retrofitted the remote release cables for the fuel door and the trunk lid, along with the fancier headlights from the DX model

The only “problem” is it’s dated nowadays. Newer cars have more power, get the same or better fuel economy, and are safer

But the car has been very reliable over the years. It’s only the spare car nowadays, but it’s never left me stranded. It’s only been routine maintenance and wear and tear items, such as batteries, brakes, cv boots, etc.

Oh, the only “problem” was when the front crank seal went. It was really pouring oil. So I changed it, along with the timing belt, cam seal, tensioner, idler, and the accessory drive belts. That was several years ago, and it’s held up

33

Hey, compared to the '91 Geo Metro I used to own, the Yaris is a luxury car.

Someone really needs to put music to this story, Commander Cody style.


I borrowed my wife’s Geo Metro last night. One liter of raw power, three cylinders of asphalt-tearing terror on thirteen-inch rims. It’s stock, alright, nothing done to it, but it pushes the barely 2000 pounds of metro around with AUTHORITY. I’m always catching mopeds and 18-wheelers by surprise…

I was headed back from Baskin Robbins with my manly triple-latte cappuccino blast (“No Cinnamon, ma’am, I take it BLACK”), when I stopped at a streetlight. As the Metro throbbed its throaty idle around me, I sipped my bold beverage and wiped the white froth my stiff upper lip. I was minding my own business, but then I heard a rev from the next lane. I turned, made eye contact, then let my eyes trace over the competition. Ford Festiva-a late model, could be trouble. Low profile tires, curb feelers, and schoolbus-yellow paint. Yep, a hot rod, for sure. The howl of his motor snapped my reverie, and I looked back into the driver’s eyes, nodded, then blipped my own throttle. As I tugged on my driving gloves and slipped on my sunglasses (gotta look cool to be fast, and I am damn cool, hence…), the night was split with the sound of seven screaming cylinders…

Then the light turned… I almost had him out of the hole, my three pounding cylinders thrusting me at least a millimeter back into my seat, as smoke pouring from my front right tire… my unlimited slip differential was letting me down! I saw in the corner of my eyes, a yellow snout gaining, and I heard the roar of his four cylinders. He slung by me, right front wheel juddering against the pavement, and he flashed me a smile as his .7 extra liters of motor stretched its legs. I kept my foot gamely in it, though, waiting for the CHECK ENGINE light to blink on in the one-gauge (no tachometer here!) instrument panel. I saw a glimpse of chrome under his bumper, and knew the ugly truth… He was running a custom exhaust-probably a 2-into-1 dual exhaust…maybe event cutouts! Damn his hot-rod soul! The old lady passing us on the crosswalk cast a dirty look in our boy-racer direction… Yet still I persisted, with my three pumping pistons singing a heady high-pitched song, wound fully out.

Though only a few handfuls of seconds had passed, we were nearing the crosswalk at the other side of the intersection, and I heard the note of his engine change as he made his shift to second, and I saw his grin in his rearview mirror fade as he missed the shift! I rocketed by, shifting, and nursed the clutch gently in to keep from bogging, keeping my motor spinning hot and pulling me ahead, now trailing a cloud of stinking clutch smoke. Not ready to give up so easily, he left his foot in it, revving, and I heard one wheel almost chirp as he finally found second and dropped the clutch. We careened over the crosswalk, now going at least 15 miles per hour. A bicyclist passed us, but intent on the race as we were, neither of us batted an eye.

He pulled slowly abreast of me, and neck and neck, we made the shift to third, the scream of motors deafening all pedestrians within a five foot circle. He nosed ahead as we passed 30 miles an hour, then eased in front of me, taunting, as we shifted into fourth. I was staring up the dual 6" chrome tips of his exhaust, snarling, my cappuccino forgotten, as he lifted a little to take the next corner.

I saw my opportunity, and counting on the innate agility of my trusty steed, I pulled wide into the number two lane and kept my foot buried in carpet. Slowly, I inched around him, feeling my Metro roll slowly to the left as I came abreast in the midst of this gradual sweeping turn. I felt the Geo ease onto its suspension stops, and felt the right rear wheel slowly leave the ground - no matter, though, because my drive wheels, up front, were pulling me through the corner, and around the Festiva …

The Ford driver beat his wheel in rage as my wife’s car eased past him on the outside, my P165/55R13’s screaming in protest, as we raced to the next light. We coasted down, neck-and neck, to the red light. I tightened my driving gloves, ready for another round, when this WIMP in the next car meekly flipped his turn signal and made a right. Chevy (Suzuki) superiority reigns!!! I drove off sipping my masculine drink, awash in my sheer virility, looking for other unwitting targets… Perhaps a Yugo, or maybe even a Volkswagen Van!

34

Not to get too far off topic about small displacement motors but Yaris, Tercel, Echo seem to be more European designed cars for crowded city driving where much of the time is spent idling and stop and go. My guess is that is where the difference lies. It’s just we Americans in love with CUBES and lots of them who like to convince our selves of everything…like bigger motors are more efficient. They are, but only if you need the capacity. A larger displacement motor bigger then necessary to do the job, has to be less efficient…everything else being equal. Man, if Toyota just junked the 1.5 and put the Corolla motor in the Yaris instead. Of course then, they would have a Scion.

@chunkyazian‌ tell me you drove the Tercel around on campus with the speed limit at 15 mph. :wink: When I drove one on the highway with a cross wind speed greater then 10 mph, it was a VW incarnate.

35

My Yaris handles crosswinds just fine, way better than the old VW Beetles would, and I know, I have driven old Beetles. Crosswind handling actually has nothing to do with engine size. Put a small block V-8 in an old VW Beetle and crosswinds will still blow that car into the next lane at 70 mph.

36

The question isn’t whether a big, powerful engine is more or less efficient: it’s what is the most efficient way to produce a given hp.


When you look at applications where size is irrelevant (like stationary power units) they go REAL big, and REAL low displacement-specific HP output. From this, you can safely conclude that big is beautiful, ceteris paribus.

(Actually, I once posted here about the “square/cube law,” and if it would be theoretically possible to manufacture an IC engine that’s simply too small a scale to function!)

37

Just look at gas mileage ratings. Those are run at fixed speeds and acceleration rates that any engine can achieve (or estimated). In a particular car, the smaller engine is almost always more efficient if multiple engines are available. If both a six and a four are available, the four will almost always get significantly better gas mileage. Check out common cars like Accords and Camrys to see the difference as their various engines use similar technologists to . Smaller engines have less friction and their cylinders have less surface area to lose heat through. The bigger engine may produce more power or torque for a given amount of fuel, but in a car you only need enough power, not too much. Having enough grunt to power an aircraft carrier is wasteful and not an advantage. It just means the engine hasn’t been sized appropriately.

The exception to smaller engines being more efficient in carsis that smaller turbocharged engines may get worse gas mileage than larger naturally aspirated engines. That’s because those turbo engines are designed to be more powerful than the bigger naturally aspirated engines. Even so, they usually burn only a little more fuel and have substantially more performance.

38

To me efficiency means how much of the fuel burned is converted into useful work and how much is lost as waste heat…Gasoline car engines range from 25 to 30% thermal efficiency. Diesels (because of their much higher compression) were able to achieve 40% and today’s common rail turbocharged designs are getting close to 50% thermal efficiency.

This thread’s title line makes the statement that large engines are more efficient than small engines. This is simply not a true statement, especially in the automotive world…

39

@dagosa‌, if you must know, we took it from the used car lot and drove it on the freeway at 70. Unlike most Toyota these days that can be steered with your pinky nail, the unassisted steering of that tercel pointed straight and had a decent feel. You would not sneeze and steer with one of those. The small engine was not burden by the power steering pump, transmission pump, or any other crap are supposed to make our lives easier. BTW, I always felt that my manual xb had better climbing power than my parents’ automatic echo. They had the same engine but the xb was 200#s heavier. The only time my xb felt anemic was when I took it from 7000 to 10000 thousand feet, climbing over the east side if Yosemite. Perhaps you should have rented a manual yaris instead. Give me a more efficient system over displacement any day.

40

This thread’s title line makes the statement that large engines are more efficient than small
engines. This is simply not a true statement, especially in the automotive world…

Definitely true. Thanks @Caddyman.