V-Twin

[Mbirt]

Do twins make a good compromise between 4cylinders and singles, balancing power and fuel economy?

On those two parameters, yes. For the power and fuel efficiency of a single but with extra weight and a CVT, you could choose the Rotax 600 ACE (no one has done it yet, but I've had a great experience with it in another SAE competition). For the power and fuel efficiency of a 4 cylinder (or worse), you could choose the Genesis 80fi. To fall somewhere in between with less weight (on par with 450-class singles) and reliability, you could choose the Aprilia 550. For compromise power and efficiency, but the weight of a 4 cylinder, you could go with a downsized SV650.

Think about this though--is more power worth more points? Check out the FSAE West 2011 results. You'll find that the same car won acceleration, endurance, and would have won fuel under the current efficiency rule.

[coleasterling]

I've always thought it would be interesting to do Ducati Supermono, but with the horizontal cylinder chopped off instead of the vertical. It would tuck nicely underneath your back and the intake and exhaust fab would be dirt simple. The engines are pretty tall and heavy, though. I haven't weighed the engines in my 620 or either superbike, but I'd guess just from picking up the 620 that they are not much, if at all lighter than the 4's.

[Z]

Do twins make a good compromise between 4cylinders and singles, balancing power and fuel economy?

Danny,

You seem to be suggesting that there is an "engine-spectrum" with the fours at one end making the highest power, but not very efficiently, and singles at the other end that can only make low power, but more efficiently. Not so...

POWER - As noted at length elsewhere, the maximum achievable IC engine power through the 20mm restrictor is about 120 hp. Also covered elsewhere is the historical fact that air-cooled, turboed, 450cc singles were putting out 120+hp over 40 years ago. So a "sporty four" has NO POWER ADVANTAGE over a single!

Note that I am not advising that you try to get 120 hp out of a 450cc motocross engine that was only designed for 60 hp max (the crank won't take it!). Instead, you would have to build your own, but that is not hard (ie. just copy above example), and FSAE is supposed to be an "engineering" competition. Also, the current fours are nowhere near that power yet, so no rush in developing said 120 hp single...

FUEL EFFICIENCY - This comes mostly from the overall car concept, and less so from the engine itself. This seen from many recent comp results where the fastest cars also used the least fuel. Briefly, DO NOT USE THE BRAKES, and instead GO FAST AROUND THE CORNERS.

Nevertheless, from an engine point of view, and for a given power output, a single will almost always be more fuel efficient than a four, because a single has less combustion surface area for wasteful leakage of the valuable heat energy.
~~~o0o~~~

Bottom line here is that based on Power and Efficiency it is draw-win to the single, at worst. The single also has a better chance of winning on engine-mass, but may have more vibration. Fitting primary balance shafts to the single will give it LESS vibration than conventional fours or twins, although this adds some mass, though potentially the single is still lightest. And overall shape, and transmission layout, are also big factors to consider...

Importantly, there is no need for a "compromise" by taking a position somewhere in the "middle of the crowd". All the advantages (for FSAE conditions) are at the single end of the spectrum.

Simple decision, IMO!

Z

[Mbirt]

POWER - As noted at length elsewhere, the maximum achievable IC engine power through the 20mm restrictor is about 120 hp. Also covered elsewhere is the historical fact that air-cooled, turboed, 450cc singles were putting out 120+hp over 40 years ago. So a "sporty four" has NO POWER ADVANTAGE over a single!

Note that I am not advising that you try to get 120 hp out of a 450cc motocross engine that was only designed for 60 hp max (the crank won't take it!). Instead, you would have to build your own, but that is not hard (ie. just copy above example), and FSAE is supposed to be an "engineering" competition. Also, the current fours are nowhere near that power yet, so no rush in developing said 120 hp single...

I'm not saying a car with 100 hp will earn more points than one with 60 hp, but there are some examples in the ATV drag racing community that prove the stock crankcase can hold together. This TRX450R made ~35 hp stock and now makes 128 hp with 15 psi boost and probably ~600 cc displacement: http://www.youtube.com/watch?v=yuXdmvreVgU

[mech5496]

Damn, that was impressive!

[Adambomb]

I feel like this thread is starting to become "What are the logistics like for the Aprilia V-Twin?" So let me attempt to return this back to my original intention. Do twins make a good compromise between 4cylinders and singles, balancing power and fuel economy?

I'll go ahead and basically echo what everyone else has said: In answer to your original question, I think Z summed it up best. When presented with the compromise between fuel efficiency and power, most teams will choose based on engine weight, ease of packaging in a chassis, reliability and parts availability.

Twins are cool, they sound awesome, but are not particularly easy to package, and really the only one that doesn't require the FSAE equivalent of 15 years (or 5 FSAE generations) of dedicated R&D to be competitive is the Aprilia, which is expensive, has a very poor starter, intensive maintenance schedule, poor reliability, and poor parts availability in the US. Also, as I recall the SV 650 engine weighs the same as most 4s, so that more or less kills it.

[RenM]

Danny,
POWER - As noted at length elsewhere, the maximum achievable IC engine power through the 20mm restrictor is about 120 hp. Also covered elsewhere is the historical fact that air-cooled, turboed, 450cc singles were putting out 120+hp over 40 years ago. So a "sporty four" has NO POWER ADVANTAGE over a single!

Wrong. With the air restrictor a single cylinder na engine will never produce as much power as a 4 cylinder, due to the fact that you can not keep a constant pressure difference over the restrictor. And please dont start with turbocharging a single cylinder for fsae. It just doesnt make any sense at all.

Nevertheless, from an engine point of view, and for a given power output, a single will almost always be more fuel efficient than a four, because a single has less combustion surface area for wasteful leakage of the valuable heat energy.

almost. In this case: not. Your heat transfer losses are a little lower with a 1 cylinder engine, but due to the fact stated above your pumping losses will be higher. Friction will be lower, but the burn rate will be slower and so on. Making such general assumptions is very difficult for a complex system like an engine.

Importantly, there is no need for a "compromise" by taking a position somewhere in the "middle of the crowd". All the advantages (for FSAE conditions) are at the single end of the spectrum.

Perhaps in your ideal world in your mind. In reality there always is. And please dont start a rant about the philosophical nature off this topic.

[Z]

RenM,

So your turboed-four will beat my N/A single, eh? Yes, of course!

But my turboed-single will thrash your turboed-four, after I remove your crankshaft! Or, putting it another way, why do you put the artificial constraint of "na" on the single design?

[Mini-Rant] (Not for RenM's eyes, but for the rest of you. )
Despite what you may think, Engineering is primarily a fashion industry.

Not so long ago everybody knew that a single-cylinder car had a snowflake's chance in hell of doing well in FSAE. That is assuming anyone was mad enough to try! Yep, you definitely needed a four, preferably turboed, to have any chance at all. And "the future" was most certainly something like the WWU V-8.

Like everything in life, more is always better.

But then a quietly-spoken team from Japan (= "TD") came to the Oz-2002 comp, and, although banned (!), showed that their 450cc single car was quite fast. Some RMIT students (= "BB") saw this, and a few years later they conquered the world. Slowly, slowly ... other raving-lunatics/rational-thinkers (take your pick) made the move. Now in 2013 the singles are regularly on the podium, sometimes without a four in sight.

But good fashion works better with a lot of complexity. Things like "engineering reasons" can go take a flying jump! So fashionable young people (like RenM?) dream up excuses for why simplicity CANNOT work, and things MUST be complicated.

For example, "burn rate will be slower" in singles, because the flame thinks "Hmmm, I can't sense any other cylinders attached to this crank, so I better slow down!". Or maybe the bigger bore (err, like the 96 mm bore of F1 engines) and slow burn-rate make the single rev slower than your ride-on lawn mower. And "pumping losses will be higher", because, err..., you can't S/C or T/C singles. Well..., unless you are an ATV-drag-racing nut, and that "just doesn't make any sense at all"! Or something else that sounds really technical...

And if you just want to be "reasonably" fashionable (ie. not at the outrageously fashionable end), then you should stay somewhere in the middle of the crowd. That "middle" used to be fours, because the spectrum extended from (ughh) singles at one end, all the way to V-8s and more (ahhh, Veyron W-16, sighhh...) at the other end. Nowadays the "middle" is probably twins, hence this thread.

But ... what engine does that GFR-2013 car run again??? [End Mini-Rant ]
~~~o0o~~~

Mbirt,

Nice engine, and I believe those power figures. With the extra bore-n-stroking I guess it also has a stronger piston and rod. My main concern would be the fatigue life of any crank with a big-end much less than about 40mm, with crank life maybe measured in minutes at 120+ hp. I reckon the simple fix is to beef the BE up to ~50 mm, and maybe also thicken the crank-webs, bigger crankcase bolts++. For the cost of a few extra kilos it should be possible to get fatigue life up to thousands of hours.

But that's just my reckoning, and I have NO PRACTICAL EXPERIENCE AT ALL (or so some say)!

Z

(PS. BTW, the generally better thermal and mechanical efficiency of a single over a four means that it should be able to extract MORE power from the air that can be sucked through the restictor. So win-win-win++...)

[RenM]

i am of course comparing a single na to a 4 cylinder na.

Thx for noting when the rant starts.

I have no where argued that a car with a 1 cylinder engine is not competitive. Dont read anything into my words, that i didnt write.
However i do argue against your 1 cylinder is better in any way. Now in 2013 singles are winning, as are cars with fours. This only shows thats there are always compromises to be made and that there is not a single best concept for fsae.

To the technical side of things: slower burn rate is a geometrical thing. Simplified the burn speed remains constant. With a larger bore the flames travel longer.
Sure you can turbocharge a single, but it wont work very well especially in fsae configuration. If you had taken your time and dig a little deeper than just the surface of this topic you´d know that in fsae conditions a turbocharger has to deal with hugely unfavourable circumstances.
The overall massflow is very low, leading to small turbochargers. Small turbochargers do however always have a bad overall efficiency.
Your exhaust gas flow is hugely pulsed as is your intake, requiring a large charge pressure.
The dynamics in an fsae car are enormous. The turbo has to be accelerated in a very short period of time. For acceleration you need power, so you close your wastegate and your exhaust backpressure rises.
Because of these points you will not be able to reach a positive pumping loop in an turbocharged fsae single, even though it is possible under different circumstances.

GFR 2013 is massively fast, because the whole package works very good together. But it does not show that a single is the only concept to go. It rather shows that there are always downsides like reliability issues.

[MCoach]

V-twin?

How much power do you really need to win this competition?