How small is too small for an engine?

[Isaac Zachary]

There seems to be a bit on confusion on how engine size affects fuel efficiency. A lot of people believe that smaller engines are less efficient because they are underpowered and will cause them to run outside of their efficiency zone, even when simply cruising down the highway at a steady speed. However, it's hard to find modern day vehicles with engines that are truely too small for the highway.

However, there have been people reporting worse fuel mileage in a small engine vehicle on the highway than with a bigger engine. But why? Of course the power needed to cruise along increases exponentially with speed. But generally a large vehicle shouldn't need more than 30 or 40hp, and a small car may need less than 20hp to sustain highway speeds. This is where a small engine should shine since gasoline engines are most efficient ususally around 75% load somewhere between 1,000 or 1,500RPM to 2,000RPM or 2,500RPM. And diesel engines may even have best efficiency even closer to 100% load at even lower RPM's. So how do you get an engine to run at it's most efficient RPM and load ranges? One easy way is to use a smaller engine.

So why are there reports of vehicles with smaller engines getting worse fuel economy than the ones with bigger engines? There could be a variety of reasons, but I think the main reason is most likely gearing. If the vehicle has a manual transmission, being at low RPM around 75% load on a flat road means the driver doesn't have much pedal left to accelerate without downshifting, especially when he or she hits a small incline and suddenly needs more than 100% what the engine can give at such low RPM's. In an automatic there'd be a bigger chance the transmission will have to shift back and forth between gears just cruising up a slight incline. So what car companies usually do is gear the vehicle with much lower gears. That way the engine stays at a much lower load, even if it's wound up at high RPM's, so that way you have a lot of pedal left to accelerate before needing to downshift.

The probelem is that high RPM's at low loads is bad for fuel efficiency, even with a small engine. In fact, it probably is better to have a bigger engine that runs at lower loads at low RPM's than a smaller engine wound up at high RPM's and low loads. Of course the best for fuel efficiency is to try to keep the engine close to it's most efficient RPM's and load. And in order to do that it helps to have a small engine but only if you have high enough gearing to keep the RPM's down.

Or just drive slower I guess. Of course driving slower with low gearing will also keep your load far lower than optimal for efficiency again.

There is an exception, and that applies to those who live in hilly areas or in constantly stop and go traffic. If you have to drive up steep roads or accelerate and want to do so at a decent speed and still get good fuel mileage you need a bigger engine in order to keep it closer to it's optimal range. Either that, or again, accelerate and drive slower with your smaller engine trying to keep it in it's optimal range going uphill. Or get a hybrid that the electric drive train helps the smaller engine when accelerating.

[cRiPpLe_rOoStEr]

A few years ago, an engineer from Mercedes-Benz claimed with Formula 1 tech it could be possible to fit a 400cc engine into a mid-size sedan. I'm sure forced induction and hybrid tech were to be considered.

[redneck]

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It’s more about the combination of components than the size of the engine.

850cc - 17hp - 85 mph top speed - 250.6 mpg achieved at the Green Grand Prix

https://ecomodder.com/forum/showthre...xed-23497.html

https://diesel-bike.com/Centurion/urba_centurion.html





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[rmay635703]

My Subaru 360 has a 360cc twin cylinder and gets mid 80’s around town if I use eoc

Seems big enough to me

Sadly everyone figures you need 4 second quarter miles and 200mph top speeds in a vehicle that seats 9 and it can’t look ugly.

[oil pan 4]

If I were to make a 5hp pusher for the leaf I would expect it to get about 90mpg at a top speed of around 45mph.
But GM did try putting under sized V6 car motors in full sized trucks, they got better city mileage but seemed to get worse highway mileage.

[Isaac Zachary]

Originally Posted by oil pan 4

If I were to make a 5hp pusher for the leaf I would expect it to get about 90mpg at a top speed of around 45mph.
But GM did try putting under sized V6 car motors in full sized trucks, they got better city mileage but seemed to get worse highway mileage.

Now they have a 4 cylinder diesel they put into their full sized one ton extended 15 passenger Vans (optional). I'm not sure what kind of fuel mileage it actually gets though.

I did once know a guy who said he three trucks, one with a V6, one with a V8 and one with a V10 and he claimed they all got exactly the same fuel mileage.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

I did once know a guy who said he three trucks, one with a V6, one with a V8 and one with a V10 and he claimed they all got exactly the same fuel mileage.

That's quite interesting. On a sidenote, were those trucks Ford or Dodge?

[ksa8907]

The real reason automakers are downsizing engines is to reduce costs. All the extra material costs money, the additional machining of a larger engine costs money, the cost to ship the engine is more per unit, the more complicated fitment of a larger engine is more costly, etc.

Fuel economy is typically not the primary concern when it comes to engine selection/design. Maybe not the most relevant example, but both generations of chevy volt were just ordinary i4's. Toyota actually upsized their engines to take advantage of the Atkinson cycle, Ford has been using the same 5.0 and EcoBoost v6's for many years now, honda has kept the same long runs of its 4cyl's and 3.5 v6 with exception of their new line of turbo i4's.

Summary: engine size is nearly irrelevant for FE, so many other factors go into the equation.

[cRiPpLe_rOoStEr]

Originally Posted by ksa8907

The real reason automakers are downsizing engines is to reduce costs. All the extra material costs money, the additional machining of a larger engine costs money, the cost to ship the engine is more per unit, the more complicated fitment of a larger engine is more costly, etc.

Can't really argue against that, even though Volkswagen's 3-cyl 1.0 TSI is still more expensive to manufacture than a naturally-aspirated 4-cyl 1.6 MSI fitted to most export versions of the Brazilian-made Volkswagen T-Cross for instance. Had it been something like replacing a 4-cyl 1.6L with a 3-cyl 1.5L instead, and both being naturally-aspirated, would've made more sense not only from a cost perspective but also due to all those claims regarding pumping losses and other factors leading to some effective impact to the overall efficiency of an engine.

[Isaac Zachary]

Originally Posted by cRiPpLe_rOoStEr

That's quite interesting. On a sidenote, were those trucks Ford or Dodge?

Ford I do believe. The guy was a bus mechanic where I drove buses.