Heres the specs on a Honda Activa scooter as you might commonly see in India or many other asian countries.
It gets 55 kmpl or an astonishing 129 miles per gallon. Doing some quick maths that means travelling 1 km only uses 18 ml of petrol, or about a tablespoon per kilometre.
Exactly how is it achieving this? How much fuel is being drawn into the cylinders with each compression cycle? It’s a 109 cc single cylinder 2 value engine, more specs on the page above.
Yeah I guess once it’s up to speed it doesn’t take much to keep it going but I still find it astonishing that 1 tablespoon of petrol has enough stored energy to move the 100kg scooter + driver that far.
So someone ELI5 (explain like I’m give) exactly how the tiny engine pulls this off? Bonus, how much more efficient is this engine than early internal combustion engines say in 1900?
How much energy is in 1 tbsp of gasoline? Quite a lot, actually. 1 tbsp is ~15 mL, and gasoline contains about 32.4 MJ/L, giving a total of 500 kilojoules in that single tablespoon. That’s enough energy to lift a 70 kg person over 700 meters straight up.
Of course, that’s spherical-cow-physics-land figuring, so in the real world you couldn’t ride a scooter up a 700 meter hill on just one tbsp of gas. As a ballpark, the efficiency of small gas motors is ~30%, and when you’re moving most of the energy is used to accelerate (and is subsequently lost when braking) and overcome air resistance. Still, as a WAG you could probably ride your scooter slowly up a 100 meter hill using that gas. Or, as we know from the fuel economy testing, you could accelerate to a reasonable speed and travel 1 km.
yeah ok I understand in terms of raw energy gasoline is a stupid amount per ml. I guess I’m asking more about the specifics in terms of keeping an engine going for that long on such a minuscule amount of gasoline. People tend to average around 40 kph on these types of scooters, so that tablespoon of gasoline is keeping the engine running for 90 seconds in order to travel 1 kilometre.
Or to put it another way, each second while running the engine is only consuming 18 ml / 90 = 0.2 ml. 0.2 millilitres is approximately 4 drops from an eye dropper…
Have I done something wrong in my calculations or is that really all it takes to keep these tiny engines running? 4 drops per second?
Remember that these figures are purely theoretical. They would apply to a scooter in perfect running condition, running on a rolling road with minimal resistance and, of course, no wind resistance. Impossible to achieve on a real road unless you have a good following wind.
The same applies to the MPG figures quoted by car makers. They only serve as a comparison.
This is an important part of the answer. The other important parts are these:
-engines tend to be most efficient at high load, and
-small engines (relative to vehicle size) tend to run at high load most of the time.
The performance specs indicate a 0-60 KPH time (that’s 0-37.5 MPH) of 10 seconds. That’s pretty sluggish, so the typical user is going to accelerate away from stoplights at wide-open throttle, providing pretty good thermal efficiency from the engine.
The V-matic transmission (which uses a metal push-belt) may also be more efficient than a conventional rubber V-belt CVT - and if well-tuned, it could put the engine at its most efficient operating point (this would be high load combined with a low/middling RPM) when the vehicle is at cruising speed.
And then there’s the matter of aerodynamic drag, of which there isn’t much. The operator’s legs are tucked in behind the fairing, so the overall profile is pretty narrow, and the whole thing is low to the ground. The OP’s link lists a 0-60 kph time, but not a 0-80 kph time, so I assume the top speed is something less than 80 kph. Contrast this with cars, for which most people evaluate the fuel economy at highway speed (110+ kph). Aero drag force scales with the square of speed, so the low cruising speed of the scooter (and its associated very-low aero drag force) is something most car drivers don’t get to enjoy.
Anything more would be wasteful, with incomplete combustion products going out the tailpipe. So for each firing you need juuust enough gas and air such that the volume of the combustion products (at some useful working pressure) matches the displacement of the cylinder. For this little scooter, the displacement is only 109 cc, which is a tiny volume – just a cylinder that’s 5 cm wide and 5 cm tall. So yes, droplets per second is entirely plausible.
Sure! If 4 drops is correct that sounds reasonable. At say 3000 RPM that would be 25 intake events/second for a 4-stroke. Without getting into air/fuel ratios I could imagine this for a 109cc engine.
I’ll agree with the previous guestimate of the modern engine being 30% efficient. But in 1900 ANY engine was a combination of blacksmith technology and science project. Intake and exhaust passages were tiny and restrictive, fuel mixing was primitive, compression ratios (which lead to efficiency) might have been 4:1 versus 9-10:1 now. There was no thought given to tuning the intake or exhaust pulses for better gas flow. Available fuel was unpredictable. So maybe 10% on a good day?
By 1930 a lot of today’s understanding of IC engines had evolved but real engines were limited by available technology and fuel quality.
the optimal air:fuel ratio for gasoline is 14.7 parts of air to 1 part of gasoline. assuming your engine takes in 100cc of air on the intake stroke, it only needs 0.06cc of fuel to go with it.
also, as has been said a gasoline piston engine runs more efficiency at higher load because you have lower pumping losses through the throttle. a little “underpowered” engine is going to run closer to its peak efficiency since you’ll be wringing out every last bit of power it has. I have a 250cc cruiser motorcycle which credibly gets 80+ mpg for this very reason.
If the displacement is 109 cubic centimeters, and the engine is operating at wide-open throttle, and we assume 100% volumetric efficiency, then each intake event draws in 109 cubic centimeters of air. At a standard density of 1.2 grams per liter, that’s only 0.1308 grams of air.
Assuming the engine operates at a stoichiometric air-to-fuel ratio of about 14:1, then that means each intake event also draws in about 0.00934 grams of gasoline.
Gasoline density is about .77 grams per cubic centimeter, so this works out to 0.012 cubic centimeters of fuel per engine cycle. That’s a cube of fuel just 2.3 millimeters (3/32") on a side.
In one liter of fuel, there are 83,333 such cubes of fuel, i.e. enough for 83,333 combustion events at full engine load. If the (four-stroke) engine is operating at 3000 RPM, that’s 1500 combustion events per minute, so one liter is enough fuel for 56 minutes of operation. If we’re cruising at 50 kph, then that’s enough time to travel 46.7 kilometers. Waddyaknow, we just came up with a fuel efficiency of 46.7 kilometers per liter, which is amazingly close to the OP’s spec of 55. Change the vehicle speed and engine load a bit, and it’s easy to see that 55 kilometers per liter is feasible.
looks like Machine Elf’s numbers are far better than mine
about 14:1 is probably right, AFAIK air-cooled bikes tend to run slightly rich-of-peak so they run cooler, at least where emissions requirements are suitably lax.
Where do you lose energy? You lose it from tire->ground drag, air resistance, drivetrain drag, and inefficiencies in the engine.
Tire->ground drag : this is directly proportional to vehicle mass. A lighter vehicle means less drag. A scooter is the lightest vehicle on the road.
Air resistance : this is a bit more complex, but scales roughly with the square of travel speed and the frontal area of the vehicle. You compare vehicles assuming they are traveling the same speed, and the scooter has a lot less frontal area, less than even a Prius.
Drivetrain drag : there’s no differential, no driveshaft. Just a chain or gear from the transmission to the rear tire. This has got to be a more efficient way to do it.
Engine efficiency : smaller engines tend to be less efficient but the difference is obviously not much. Honda must have installed modern fuel injection with sensors and computer controlled injectors (it’s a really small computer). No accessories to drive as there is no power steering, no air conditioning, no power brakes, very few lights, so no losses there. Air cooled engines are also inherently more efficient as there’s no pumps to move the coolant around.
So there you have it. All these factors sum together to really save on fuel. Only problem - the chance of getting hurt is a lot higher when you don’t have a whole vehicle around you. I suspect that even in the third world, the cost in lost income and medical bills probably exceeds the money you save on fuel…
This is how you analyze this. It’s not about how much energy it takes to accelerate a vehicle to speed, or how much gasoline the engine can possibly accept, it’s about how much energy the vehicle loses while traveling and inside the engine/drivetrain. (especially since it is possible with regenerative braking to get energy spent accelerating the vehicle back, but you can’t get energy back you lost to friction)
I can’t speak for scooter test conditions, but I can tell you that the MPG figures on new-car window stickers are measured under very tightly specified test conditions that do factor in real-world rolling resistance (with tires inflated to the spec on the sticker affixed to the driver’s door jamb) and aero drag. If a manufacturer doesn’t adhere to the required test conditions - and they get caught - they get nailed to the wall.
My Honda CX500 at highway speed would get about 240km on 3 gal (12l) or about 20km/l.
this is at highway speeds, with a much larger cross-section on a 1979 motorbike with standard carburetors but electronic ignition. My 1990 Honda Civic with a 1600cc engine and electronic fuel injection could typically get 45mpg highway, about 15km/l at 55mph.
(Numbers off the top of my head, and may be confusing 3.8l US gallons and 4.5l Imperial gallons).
Going even further back, a 49-cc Peugot 2-stroke moped I had in the mid-70’s would get close to 100mpg with a top speed of just over 30mph…
Modern electronics can adjust timing for ignition far better than was possible before with mechanical ignition points - this ensures more timely and complete firing of the mixture. Fuel injection (which simple scooters like this don’t have) would be even more miserly with fuel, ensuring that very little went out the pipe as unburnt or partially burnt fuel.
This is quite obviously not true or there wouldn’t be 100’s of millions of people riding tiny scooters like this all over Asia. It’s true that accidents are common, but due to road congestion they tend to happen at low speeds, so mostly minor scrapes and bruises. Also most of these countries have socialised medicine so the bills are not high if someone does need medical care.
Nope, it’s just got a carburetor and a basic CDI ignition setup. There are zero computers on the thing. There’s a few clever refinements here and there, but this scooter is the same general idea mechanically as what Honda has been cranking out since the late 50’s. Even back then they got well over 100 MPG. (Honda used to claim over 200 MPG for the original 50cc SuperCub, which was definitely optimistic but not wildly off the mark.)
It’s really more remarkable how little small motorcycles and scooters have changed over the same decades that have lead to revolutionary improvements in cars. Once you’re in the triple-digit MPG range, the returns rapidly diminish in terms of how much you’re actually going to save in fuel over the expected life of the bike, which doesn’t leave a lot of room for high tech fuel-saving technologies to pay for themselves.
What I wonder is how well the current EPA city & highway test profiles reflect conditions people actually experience when they drive. E.g. if the EPA highway profile is to accelerate very, very slowly to 50mph & hold that steady for 3 hours then coast to a stop, all the while on dead level low-friction smooth roadway with zero wind other than that caused by vehicle speed, then Yes, a manufacturer could administer the test precisely and fairly then honestly report the results. But at the same time, No, I won’t get that many MPG when I drive on the freeways around here in real world traffic.
Do we (trade press, enthusiast press, smart messageboard guys, etc.) have any commonly accepted metrics for how “real world” the current EPA tests are and aren’t? IOW, what’s the conversion factor from test to reality?
Not really, because obviously those metrics would be completely different for every driver. The ratings are only meant to be a good basis for comparison between two cars, not necessarily absolutely accurate.
There have been some arguments that some cars are much better at the EPA test cycle than most people’s actual driving habits (that’s often leveled at hybrids) but in general the system works pretty well. If Car A’s MPG rating is 20% higher than Car B’s, there’s a pretty good chance Car A is going to be 20% more efficient regardless of what actual numbers you’d personally get with both cars.
YMMV.
(And, somewhat relevant to the OP, there isn’t any similar standardized test for motorcycles in the US so the situation with them is still very similar to what it was for cars before the standardized EPA tests. You still see motorcycle manufacturers putting some pretty crazy numbers in ads.)
