How big a capacity could you make a flat twin engine?

Some of you may know my obsession with trying to design small, light road cars as I think they make more sense than 500 bhp 1,800kg supercars.

The basic idea is always the same -cut weight and drag by reducing the plan area whilst maintaining a normal track/wheelbase ratio. If you can do this  its "win/win" - as the size goes down the weight falls so narrower tyres are OK which reduces width and saves drag etc.

There are two big problems -the occupants don’t shrink pro rata and as you narrow the track you really should reduce the CG height as well to keep roll angles versus spring rates in balance. Also as you reduce the wheelbase the pitch sensitivity goes up unless you lower the CG.

One way to help solve these problems is to use rear flat cylinder engine like a beetle/beach buggy/ Porsche 911. This has very low CG and its position allows all the wheelbase to be used for the occupants who can be reclined to further reduce the drag and CG. The problem then becomes excessive rear end mass hung outside the wheelbase.

So I was thinking that if you could build a really large capacity flat twin you could get enough power - say 200 - 250bhp with a low engine mass very close to the rear axle centreline.

 So how big could you build a flat twin ( in principle ) before the pistons etc weight limits rpm and induces excessive side shake due to the offset crank pins?

The new Morgan 3 wheeler uses a US Vee twin of about 1.8 litres, The Lycoming IO 580 aircraft engine is 9.56 litres/583 C.I. from 6 cylinders. So if you used the Lycoming engine as a reference you could have a 3.2 litre flat twin. Using the Lycoming dimensions it would have 135mm bore and 111mm stroke.

The aero engine is obviously heavily de rated for safety so it runs at only 2,500 rpm.

My question is could such a big flat twin runs successfully and would it be able to run up to , say, 5,000 rpm or so?.

I know the drag racing “monster motor V-8's have over one litre per cylinder and wind to serious revs but with a short life.

Any thoughts please?

Edited by mariner, 10 January 2014 - 15:10.

Some of the Japanese cruisers have pretty big V-twins - Kawasaki have one that is a hair over 2L.

https://en.wikipedia...can_2000_series

Would a boxer really have a CoG much lower than a wide-angle V once you allow enough ground clearance for the exhaust? Are you designing for much suspension travel? Will approach/departure angles become a problem with a boxer?

BMW's 1200cc boxer engines are surprisingly tall - as tall as the ~990 V-twin in my KTM. There's substantial mass both above and below the cylinder jugs. I suppose you could shrink the bottom end with a dry sump arrangement.

There are people out there building larger V-twins (some up to 3.5L) to be installed in silly motorcycles, but I haven't heard of any really big flat engines other than the aero jobbies you already mentioned. And no big flat twins at all that I've seen. I am sure they are out there though.

You didn't specify a target torque figure but KTM now have a 1301cc V-twin with a warranty, ~180bhp, 141Nm and supposedly an engine weight (I assume this includes gearbox & clutch, since they are a combined unit...) of 62kg. Dry sump.

Edited by indigoid, 10 January 2014 - 13:12.

There is that awesome JAP V-twin engined GN hillclimb car, from memory that is a 4.3 litre. Sounded lovely before the noise regulations.

Don't be put off by the above mentioned height of the BMW bike engines. Although they are high, that is because they are bike engines: the cylinders have to be so high to allow decent lean angles and then the sump depth doesn't matter, the space above the crankcase is quite large in a bike of this layout, so stuff is fitted in the space.  A good thing about a flat TWIN is that exhaust ports can exit forwards and / or rearwards (unlike, say, a 911) so the engine centre of mass could be really low. Yours would need to be dry-sumped of course. 

Do it, the world needs this!

this is the Lycoming engine.

http://www.lycoming..../580Series.aspx

it is 39" long with all six cylinders so just the end two cylinders and the  flywheel it  might be about 15" or375 mm long . Its 21" high with a big wet sump so maybe 11"/ 275mm high with a dry sump.

Honda goldwing got a 2Liter engine with reverse. All you need in one package. Just get a gear where the wheel was and a diff to match further down.

If not you have to consider big harley twins. 2L+ and such but they costs more than it goes. But good part supply and very tuner friendly. If only lightly tuned a stock ducati engine and such is probably better given the lighness and hp.

I recently thought about a ducati powered Fiat 500.. as they weight in at 500kg stock. My ponderings had the reverse as a simple electric motor. (Hybrid! )

Edited by MatsNorway, 10 January 2014 - 16:03.

Wasn't the Goldwing reverse gear just an electric motor?

haha! your right!

seems like thats how it goes. I like it when my ideas are in use and proven valid.

http://answers.yahoo...14132147AAU74YP

Edited by MatsNorway, 10 January 2014 - 19:22.

Wasn't the Goldwing reverse gear just an electric motor?

Yeah I think so. Also a horizontally-opposed 6, not a twin, and 1800, not 2000. Great engine though.

Another option - BMW K75/K100 engines were a laid-down I3/I4 respectively. Cutaway looks like this:

Couldn't you just turn the exhaust port about 60 to 90 degrees towards the rear? That would reduce the depth of the package.

Mariner, you are asking a question some what like, "How long is a piece of string?"  This because you have set no parameters as to the performance standards you wish to meet with the type of (unspecified) vehicle you are aiming at.

On one hand, if you are planning on a four seat sedan and wanting 300,000 km life and a marketable package you come up with one type of answer that might be up to about 2 liters max fitting your bill.

On the other hand, if you are planning on a two seat road usee sports car where performance is more important but a bit of vibrational character could be a selling point you might be looking at 2.5 to 3 liters.

If you want pure performance such as track use with say 10 hour engine life you can think in terms of pretty radical sizes and such as applying some form of supercharger/turbo and start pushing beyond the 2.5 liter capacities. Balance capacity versus induction pressures.

Regards

The idea is just a "what if" sort of study. I envisage a car one step up from a Caterham seven but cheaper thana supercar. Backbone chassis - one thought is to have that in steel and use the rear section as the silencer box to save on the usual weight/bulk of a seperate silencer.

The engine life would be , say 200  hours between overhauls - that might give 4-5 years of service for  track/special days out usage.

I've found out that you can buy an entire head/barel/piston asembly for one of these engnes here

http://www.aircraft-...540-tisn10-1ca/

Edited by mariner, 11 January 2014 - 18:14.

 

I've found out that you can buy an entire head/barel/piston asembly for one of these engnes here

 

http://www.aircraft-...540-tisn10-1ca/

The injector position appears to be pointing out the port ?

I think around 1.8L to 2.0L would not be too outlandish with balance shaft and isolation mounts. 

The injector position appears to be pointing out the port ?

I think that is the spark plug lead- on aero engines they are supported in metal tubes

VW just advertised a 4-cylinder Passat concept car that uses cylinder de-activation to achieve a highway fuel economy of 42MPG. The car de-activates 2 of the in-line engine's 4 cylinders. This would technically make the engine a "360 deg flat twin".

http://www.greencarc...passatblue.html

Makes about as much sense as the "360 degree" cranks in old Triumph vertical twins.

VW just advertised a 4-cylinder Passat concept car that uses cylinder de-activation to achieve a highway fuel economy of 42MPG. The car de-activates 2 of the in-line engine's 4 cylinders. This would technically make the engine a "360 deg flat twin".
 
http://www.greencarc...passatblue.html

The idea is just a "what if" sort of study. I envisage a car one step up from a Caterham seven but cheaper thana supercar. Backbone chassis - one thought is to have that in steel and use the rear section as the silencer box to save on the usual weight/bulk of a seperate silencer.
 
The engine life would be , say 200  hours between overhauls - that might give 4-5 years of service for  track/special days out usage.
 
I've found out that you can buy an entire head/barel/piston asembly for one of these engnes here
 
http://www.aircraft-...540-tisn10-1ca/
NEVER use the chassis as an exhaust system. Apart from it being generally impractical it will burn out the chassis and weaken or break welds.
The Clubman style tube chassis type of car should be fun, there is many commercial styles available or get your tape measure and camera out and build your own. If you wish to use a flat engine use the obvious, a Subaru. The larger bike engines are probably viable too.Should be considerably lighter. Though far more power available from the Subaru, even as a normally aspirated engine.
A bigger VW with mostly aftermarket parts is an option too. Though VERY dated. And get heavy with the aftermarket parts needed to make them live.

http://www.carscoops...der-105-hp.html

http://en.wikipedia....iki/Ducati_1199

At the time of its release, the Ducati 1199 Panigale was the world's most powerful production twin-cylinder engine with maximum 195 horsepower (145 kW) at 10,750 rpm and 98.1 lb·ft (133.0 N·m) torque at 9000 rpm.

Edited by scolbourne, 13 January 2014 - 08:18.

Well, if by current showroom road-bike that may be correct, but I'm fairly sure there are numerous after market twins

available in the USA that can better that..

& if specific output/power density is the parameter being measured, then the last Aprilia RS 250 roadbikes were

good for ~70hp, & their  RS 250GP race bikes ~110hp..

Interesting post question though, I recall looking at some Bristol Hercules sleeve valve cylinders & thinking about a twin.

& a local bloke did slice a V twin section off a 27 litre RR Meteor/Merlin mill to build a bike - a while back..

Well, if by current showroom road-bike that may be correct, but I'm fairly sure there are numerous after market twins
available in the USA that can better that..
 
& if specific output/power density is the parameter being measured, then the last Aprilia RS 250 roadbikes were
good for ~70hp, & their  RS 250GP race bikes ~110hp..
 
Interesting post question though, I recall looking at some Bristol Hercules sleeve valve cylinders & thinking about a twin.
 
& a local bloke did slice a V twin section off a 27 litre RR Meteor/Merlin mill to build a bike - a while back..

'Any thoughts please'

Well..

This one may be no bulkier/weightier, than a flat twin - but 3 perform better than 2.

http://www.4x4tuff.com/ctsme8.html

Edited by JAW, 13 January 2014 - 23:22.

I doubt Mariner's concept could utilise a 2 stroke.

Why not?

Banned in motorsports like F1 & Moto G.P.- sure because 2Ts are just too good,

but if he wants a prototype, that's up to him..

Sir Stanley Hooker of R-R aero-engine fame summed it up - something like this..

"4-strokes  = 1- stroke  - for power,

& 3 - to wear the engine out".

Edited by JAW, 14 January 2014 - 00:14.

Most anything they bother to ban in motorsports will be a very good thing for going faster else they wouldn't bother banning them.  Motorsports design solutions are thus often a lesson in what not to logically do.  And the bigger the rulebook, the further from optimal design the resulting solutions that conform (and hence more ridiculous the resulting cars from a design standpoint) will be. 

Isn't Mariner talking about supercars ie road going?

Prototype lightweight proposal - as alternative to big heavy supercars, isn't it?

'Spose he could run a 2T - on the road as a 'modified' DKW/SAAB or Trabant for rego purposes.

Edited by JAW, 14 January 2014 - 00:57.

Desmo is correct about ideology trumping logic technical performance-wise.

In motorsport anyhow..

Honda would love to market a 4-stroke chain-saw,

-  but to compete with a 2T..

..it'd have to be turbo-charged, dry-sump, Ti/Magnesium,liquid-cooled unit that would likely be.. a bit pricey,

& averse to sitting around in a shed for months between bursts of hard work..

The OP posed the question "how big a capacity could you make a flat twin engine?".  The theoretical answer is that there is no fundamental limit to how large the displacement of a flat twin engine could be. If you were so inclined, it should be possible to build a flat twin recip engine with a bore of 4 feet and a stroke of 8 feet. Of course the engine would be incredibly heavy and would run at a speed of around 100rpm or less. But its power and SFC numbers would be awesome.

If you were proposing a spark ign mill that big, you'd need some fairly comprehensive DFI/engine management installed.

The flame front burn speed is gonna be wanting multiple injectors/spark plugs - all timed to the nth degree..

Yeah, that FIAT was a beast, & some other ex-aero engine powered cars are intense vehicles too,

that Napier-Railton being pounded, getting air - around the Brooklands banking by John Cobb springs to mind..

I think that the topic is looking at the opposite tack, however..

Aero-engines are built light/under-stressed though, I recall seeing a flat-6 aero engine powered,

' special ' being raced as a kid, in `60s NZ..

These days, ex-motorcycle engine/gearbox units are popular for small/light racing car classes,

just as formula 500/1000 Coopers ran, back in the day..

If you were proposing a spark ign mill that big, you'd need some fairly comprehensive DFI/engine management installed.

The flame front burn speed is gonna be wanting multiple injectors/spark plugs - all timed to the nth degree..

Wartsilla builds a 4-stroke SI engine with a 500mm (19.7 inch) bore. It seems to work quite nicely.

http://www.google.co...1,d.cGU&cad=rja

Got one to show  - that will  'work quite nicely'  - running a 48in bore - as previously suggested?

 (In your post # 30, B-L-S)

Now that would be something..

Got one to show  - that will  'work quite nicely'  - running a 48in bore - as previously suggested?

 

 (In your post # 30, B-L-S)

 

Now that would be something..

JAW- No, I cannot provide a specific example of an SI engine using a 48" bore.  However, I only claimed it should be possible in theory.  If an SI engine with a 19.7" bore can be made to work satisfactorily, then I cannot see any reason an SI with a 48" bore could not also be made to work.  But just because something might be possible to achieve does not mean it would be a good idea.

There is very likely a practicable limit..

... to feasible flame-front/combustion stability for S.I. mills when bore sizes are vastly scaled-up B-L-S..

This would tend to rule out a machine thus equipped to provide useful work..

.. efficiently,  by comparison to current 2T Diesels..

Wartsilla builds a 4-stroke SI engine with a 500mm (19.7 inch) bore. It seems to work quite nicely.
 
It also has an electrical output efficiency of 48.6% - implying probably that the actual TE of the engine is over 50% - very high for an SI Otto-cycle engine.

Edited by Kelpiecross, 21 January 2014 - 09:05.

I think I've solved my own question by accident .Today I saw a Subaru flat four stripped to the basic blocks and heads for the first time. I had not appreciated how short it was flywheel to cambelts. Just what my design needs.

I think it is so short because its water cooled so none of the big airflow gaps betwen cylinders on air cooled VW or Porsche needed.

I have explain how I came to see the engine on stand as its a story you couldn't make up. I am organising a tour of the historic Lotus sites in North London and Hertfordshire ( just north of London). Many sites are likely to be demolished soon as they are now in very poor industrial araes. One such site is th original home of Cosworth in Edmonton, North Lonon.

I went there today to check it was till standing which it is. When I asked if I could take a photo and explained it was a place of motor racing history I was beckoned inside the old Cosworth building where a nice Bulgarian gentleman showed me his Subaru engine building operation!

I told him of its history and that he had very high standards to live up to

He seemed interested in Cosworth, and had used some of their parts in the past so I hope he can keep building Subaru and BMW engines for Bulgarians there for some time

Edited by mariner, 30 January 2014 - 18:05.