Engines - What if...

[SoulKiss]

As a spin off from the Radial Engine thread (great reading and WOW! at some of the designs) I thought I would start this thread for questions on engines.

How they work, why things are done in certain ways, that kind of thing.

So let me kick it off.

What difference does the length of Crankshaft make?

Assuming the same combustion chamber volume, what is better, a short fat chamber or a long thin one?

Over to the engineers out there for answers and the interested for more questions.

[Spank86]

I was always under the impression that with regard to the chamber a square one was best, (so the length of travel is equal to the circumference of the piston), but ducati appear to disagree and you cant argue with results.

[jambo]

What would you like your motor to do?

If you want maximum power, then you need to get your valve-piston area as high as possible (meaning use as many cylinders as you can reasonably package in the space). You also need to rev the motor, necessitating a short stroke and wide bore to reduce the stress on the crank & con rods. The down-side? You'll have poor characteristics at low rpm, as the fuel-air mix at low engine speeds will be traveling quite slowly through those large intake chambers, leading to poor mix & swirl, and poor volumetric efficiency.

Want something to produce the most torque possible at low rpms? You'll want a longer stroke, and smaller bore, so that the speeds of fuel/air coming through the inlet chamber is high at lower rpms.

It's about where your engine makes it's peak torque, which is down to volumetric efficiency.

Jambo

[Small Clanger]

There's a very interesting article on www.thekneeslider.com (http://www.thekneeslider.com) about the 1199 Ducati Panigale oversquare engine.

Type Panigale into the search box.

:)

[Sid Squid]

Just to clarify: What do you mean by 'length of the crankshaft'?

If you mean the length of the piston's stroke, then there are a number of considerations, (as Jambo has said), which are based around the question; what do you want your engine to do? There is no 'best' solution to the question of those compromises it's entirely dependent on the purpose of the engine, and indeed, which type of engine; four stroke, two stroke, diesel, Wankel etc. etc.

All else being equal, and given some basic assumptions, say a cylinder of a given swept volume, there are some very general things that basic engine proportion will dictate:

Long stroke and narrow bore, often referred to as 'undersquare'.

Good: The crank has a longer throw, thus in essence the 'handle' that the engine turns is longer and easier to turn, (just like a long breaker bar is easier to use than a short spanner when loosening a tight bolt), this greater mechanical advantage translates to more torque. Intake phase is lengthy and gas speed through relatively small amount of valve area is higher, thus volumetric efficiency, (VE), is good at lower speed improving torque.
Bad: Narrow cylinder means less cylinder head area for valves, limiting their ability to pass gas in and out, and less piston crown area for the gas to push on. For any given engine revolution speed the piston speed is higher, (longer stroke = more distance, more distance in the same time = more speed), so as engine speed rises the ignited gases are trying to press on a piston that is moving away from them at a higher speed limiting the effectiveness of the push they can provide. Also there are material constraints limiting safe peak piston speed which will be reached at a lower engine revolution speed, and mechanical stresses of are increased.

Short stroke and wide bore, often referred to as 'oversquare'.

Good:
Wide bore allows lots of area for valving and piston crown area for the gas to act on.
Short stroke means the piston speed is much lower, meaning the engine can be turned faster before the material limits and mechanical stresses are reached.
Bad: Timing is critical as the intake phase is short and if the possible advantage of more valve area is taken the gas speed through larger valving is lower which can impact badly on VE at low engine speeds, the short handle the engine turns has less mechanical advantage which translates as less torque.

The above is an extreme simplification of the circumstance and discusses only the two extremes, hopefully it makes some of the compromises of engine design easier to appreciate.

[yorkie_chris]

Engine power is all about moving a mass of gas through the engine.

You need 14.7 kg (so about 15 cubic metres or 15,000 litres, or about a room full) of air to burn every 1kg (1.2 litres... ish) of petrol.

If you want power you need to burn petrol, which needs air.

To move lots of air you need BIG valves and lots of them. Or there are various whirlythings (superchargers and turbo-superchargers) to mechanically stuff more air in. Or nitrous, which basically works by artificially adding more oxygen, allowing more fuel to be burnt.

will be traveling quite slowly through those large intake chambers, leading to poor mix & swirl, and poor volumetric efficiency.

No reason to not get very good VE with this layout, what tends to knacker things for snorting high performance engines at low rpms is the cam timing... which is set with lots of duration (for gas flow at high rpm...) leading to air being shoved back out of the cylinder before the valve closes.

[embee]

When designing car engines there are a lot of issues to consider, not just performance.
Obviously output, but then fuel consumption of the vehicle, emissions (regulated pollutants not just CO2 which is fuel cons by another name), weight, package size, reliability and durability, NVH (refinement), servicing, cost etc. Secondary issues like commonality of parts across ranges etc are also considered.
As for performance, as said by others, an engine is essentially an air pump, the air (oxygen) allows fuel to be burnt and heat released, the rate at which heat can be released and how efficiently it can be turned into mechanical effort gives the potential power output.
In simplest terms the power is limited by the aerodynamics of the air flow into the cylinders, once starts to reach sonic speed through the throat (at the inlet valve/seat) it begins to choke and the flow rate becomes limited, that's what gives the max power speed of the engine. Broadly the more inlet valve area the greater the flow rate of air.
Chamber types have fairly well established empirical valve sizes relative to bore diameter, a 4-valve pent-roof chamber will have inlet valve diameter Di about 0.35xB. Any bigger and they start to become shrouded by the walls and each other so flow gets restricted. The thickness of casting sections also influences things, and cooling of spark plug or injector bosses need to be considered.
Having said that gas speed is the limiting factor, it's not an easy parameter to work with, and a useful term which can represent it is "mean piston speed". The MPS at which max power is reached is fairly typical for a given chamber type, due to the more or less typical ratio of valve area to bore area. For a 4-valve with Di=0.35B, the area ratio is about 4 (Ab=4Avi), so a representative gas speed is around 4x representative piston speed. When you boil it all down (include discharge coefficients and mach index etc) significant choking starts and thus max power comes in around 20m/sec mean piston speed. Modern engines achieve a bit better, some around 22m/sec now, but it's all in that ballpark. Direct injection helps in that fuel doesn't take up inlet gas volume, so air flow capability gets extended. 20m/sec MPS equates to 12000rpm with a 50mm stroke, or about 9500rpm with 62.6mm stroke like the SV650.
The shorter the stroke the higher the RPM at which you reach the choking, and the higher the potential power for a given swept volume.
Remember that this mean piston speed is not a mechanical limit, it is a representation of the aerodynamic limit.
Big bore sizes however lead to high surface to volume ratios at TDC so lots of heat loss, smaller bores tend to permit better thermal efficiency and also usually higher knock limits (limiting compression ratio).
Traditional multi-V engines (V8 etc) had big bores to make use of the space given by the extra conrod width on shared crankpins, but modern ones tend to be smaller bores for efficiency. In line car engines tend to have quite small bores these days for packaging and efficiency reasons.
Torque is primarily a function of swept volume, secondarily the chamber type and thermal efficiency (comp ratio etc).
The "specific torque" (or Nm per litre) is a unit of pressure, which is conveniently expressed as Brake Mean Effective Pressure (BMEP), it's the average (or mean) pressure which if it was acting in the cylinder during the expansion stroke of a frictionless engine would have the same effect on a brake (trying to stop it turning) as the real gas pressure does in a real engine. A really good 4-valve will produce around 12Bar BMEP at max torque these days, more typically you'll see around 11Bar, the conversion to torque is given by
Nm = BMEP x V x 7.958 (the 7.958 comes from 100/4Pi when taking radians and convenient engineering units into account)
so a 650cc engine producing 11Bar BMEP would give 57Nm.
BMEP allows very different engines to be compared directly, the size and shape of the engine (number of cylinders etc) is taken out of the equation.

There is more....................

[punyXpress]

Back in the 60s a pal of mine designed a variation on the IC engine where instead of minimising losses due to side thrust of the piston, he MAXIMISED it - a variation of the radial although his was single cylinder.
Patented, but never heard more of it so suppose he's still at it!
Also lost touch about 1970

[Sid Squid]

There is more....................
Yeah yeah yeah - and just exactly whaddya you know about it then?

Armchair experts eh?

;)

[embee]

Make it up as I go along usually, it's got me by so far. :smt073

[littleoldman2]

Just to throw in a bit of fun. Most current automotive engines are OHC or DOHC engines. Mostly due to engineers designing an engine of ????cc for taxation, racing formula or legislative reasons, add to that the marketing influence. What type of engine would we see if power, reliability, efficiency, size, weight and in the case of motorcycles handling (in any order of priority) were the real factors.
What I'm saying is instead of a 600cc supersport class we had a xxxbhp / xxxkg supersport class. If the answer to the question "how big's the engine" was in power, torque, size or weight instead of capacity then would we have very different engines to today. Side valve SV anyone?.

[yorkie_chris]

Probably DOHC, or rotary.

Side valves are horrific in terms of thermal efficiency, gas flow, flame behaviour and everything else really. Except ease of manufacture maybe.

[littleoldman2]

I assume you mean the Wankel engine as opposed to lots of cylinders whizzing round. This Wiki article illustrates my point about cc and taxation / sporting formula's http://en.wikipedia.org/wiki/Mazda_Wankel_engine.
DOHC I can see no reason for them in an engine not limited by cylinder capacity, just use OHV or anything else to get rid of the size, weight and complexity, then replace it with bigger pistons at lower revs making the same power.
OHV (& 2 strokes etc.) also frees up thinking about where you can put the cylinders as you need less cams, 12 cylinder radial engine, DOHC = 24 cams where as OHV = 1 or 2 cam.
I think If we were not limited by power not capacity then the "sport" class would be something like a 200bhp liquid cooled V4/6 OHV of around 2Lts, or a radial 4 cylinder 2 stroke, possibly Wankel if you can put up with the poor fuel consumption.

[yorkie_chris]

So you would get rid of size and weight... and replace it with size and weight with a big numb engine?

In a bike you need high specific power for performance, good packaging. There are only a few that readily fit.

Look at V4, like the RSV4. About 200hp from only 1l. If you replaced that with an OHV engine... like the conveniently comparable ford essex, OHV V4 of about double the capacity it made only half the horsepower and weighs more than the bike!

Bikes are pretty evolved, I think we'd see the same configurations.

What I would like to see is some like 250/400 turbo IL4s powering sensibly sized bikes. That could be interesting.

[andrewsmith]

Bikes are pretty evolved, I think we'd see the same configurations.

What I would like to see is some like 250/400 turbo IL4s powering sensibly sized bikes. That could be interesting.

We've been there before, albiet they were big heavy Garrett blowers.

You thinking along the lines of the Holeshot Racing bikes?
http://www.holeshot-racing.co.uk/index.php/services/turbo-charging/14-yamaha/47-r6-turbo-06-11

http://www.holeshot-racing.co.uk/images/turbo-charging/yamaha/r60611/vsig_images/R6t060720101574_788_591_90.jpg

[yorkie_chris]

We've been there before, albiet they were big heavy Garrett blowers.

You thinking along the lines of the Holeshot Racing bikes?
http://www.holeshot-racing.co.uk/index.php/services/turbo-charging/14-yamaha/47-r6-turbo-06-11

No not at all.
Not big stupid ballistic missiles which are unusable for anything.

But imagine commuter bikes that still make reasonable power, but have very lazy cams and intake design for good economy and torque.

[andrewsmith]

No not at all.
Not big stupid ballistic missiles which are unusable for anything.

But imagine commuter bikes that still make reasonable power, but have very lazy cams and intake design for good economy and torque.

I know what you mean, just linked that as a technical example (at the stupid end of the scale).
It'll depend on how small Mitsubishi and the others can compact the technology without making it a grenade. The unit holeshot use is one of the smallest in production (nicked of a Colt CTZ engine)

Sounds like a good use of an ER4 if they ever get round to importing it

[littleoldman2]

What I would like to see is some like 250/400 turbo IL4s powering sensibly sized bikes. That could be interesting.
Oh yes please, so would I but maybe not IL4s and add a turbo or supercharger.

Bikes are pretty evolved, I think we'd see the same configurations.


Yes they have evolved in a capacity limited environment. Change the environment and the evolution would have been different.

So you would get rid of size and weight... and replace it with size and weight with a big numb engine?

With current technology and materials there is no reason for them to be big or numb. Just shorter with the weight low down where it belongs. With respect to the Ford Essex engine, the same argument can be put for any car IL4 with DOHC as well. Look at V4, like the RSV4. About 200hp from only 1l and think what the designers could do if the brief was "same size and weight" with no other limits, nitrous maybe.

Fact is the governments have dictated engine development for their own ends. As for the racing influence just look at the daft "diffuser" like rear ends on family cars.

[yorkie_chris]

I'm not going to discuss any politics TBH, I'll stick with technical stuff where I belong.


So given the fact that an engine is an air pump, and that it develops power by burning fuel at a relatively fixed ratio to that air, it follows that the only way to generate power is to flow air. The more air you can flow the better.

So given this do you think it is better to "spend" the weight you have on the cylinder head or the barrels?

[littleoldman2]

As a lover of 2 strokes its cylinders every time, but will have to wait for the engineers to develop a clean 2 stroke. I haven't the knowledge with 4 strokes and the current racing formulas give us no an hint, having said that look at WSBK 1000cc IL4s vs 1200 twins I wonder which is the smallest and lightest of those two.

[punyXpress]

How's about forgetting swept volume & going Turbine?

http://www.theengineer.co.uk/Pictures/web/y/c/g/Bladon-Jet.jpg


Read more: http://www.theengineer.co.uk/in-depth/analysis/jet-power-bladons-microjets-enable-jaguar-turbine-hybrid/1005528.article#ixzz1wvG54qcJ

[littleoldman2]

How's about forgetting swept volume & going Turbine?

http://www.theengineer.co.uk/Pictures/web/y/c/g/Bladon-Jet.jpg


Read more: http://www.theengineer.co.uk/in-depth/analysis/jet-power-bladons-microjets-enable-jaguar-turbine-hybrid/1005528.article#ixzz1wvG54qcJ

Very interesting and from the write up runs on almost anything.

On the 2 stoke front http://thekneeslider.com/archives/2011/09/28/jj2s-x4-500cc-2-stroke-update/

[yorkie_chris]

Aye turbines are pretty fuel tolerant, all it has to do is add heat.
No worries about detonation resistance (octane) or ease of ignition through heat and compression (cetane rating, diesel engines).

2 strokes are a laugh but who has them as a reasonable alternative to a modern 4 stroke? The smell is ace, but they're dirty. They've also got terrible volumetric efficiency except when they come up onto the pipe, which means a (very entertaining) terrible power delivery. The poor lubrication makes them unreliable.

That bike is really cool, but how is it going to compare with usual engines? It will still have 2 stroke problems,.

[yorkie_chris]

This is worth a read
http://www.feralinjection.com/

[Scythe92]

Aye turbines are pretty fuel tolerant, all it has to do is add heat.
No worries about detonation resistance (octane) or ease of ignition through heat and compression (cetane rating, diesel engines).

2 strokes are a laugh but who has them as a reasonable alternative to a modern 4 stroke? The smell is ace, but they're dirty. They've also got terrible volumetric efficiency except when they come up onto the pipe, which means a (very entertaining) terrible power delivery. The poor lubrication makes them unreliable.

That bike is really cool, but what is special about the engine? Do it because you can, sure, but what's new in there?

YC has it in one. 2 strokes just can't be made 'clean'.

[littleoldman2]

I think the special thing about the engine is in the common crankcase and using the cylinder instead of the piston as the moving part * to achieve the crankcase compression needed. This also allows the crankshaft and crankcase to be lubricated in the same way as a four stroke, giving cleaner running. BTW couldn't agree more about the old 2 strokes.
* Or integrating the piston into the cylinder
The possibilities are there it just needs something to drive development after all we wouldn't have diesel engines if German colonies had oil instead of peanuts.

[grimey121uk]

Aye turbines are pretty fuel tolerant, all it has to do is add heat.
No worries about detonation resistance (octane) or ease of ignition through heat and compression (cetane rating, diesel engines).

2 strokes are a laugh but who has them as a reasonable alternative to a modern 4 stroke? The smell is ace, but they're dirty. They've also got terrible volumetric efficiency except when they come up onto the pipe, which means a (very entertaining) terrible power delivery. The poor lubrication makes them unreliable.

That bike is really cool, but how is it going to compare with usual engines? It will still have 2 stroke problems,.

That true regarding the typical 2 stroke as we know them but there is no reason why a 2 stroke couldn't have overhead valves, fuel injection and a dedicated lubrication system. In fact look at modern ships, they use 2 stroke turbo diesel engine for efficiency and power.

[punyXpress]

The possibilities are there it just needs something to drive development after all we wouldn't have diesel engines if German colonies had oil instead of peanuts.
Compression Ignition engine invented by : Herbert Akroyd-Stuart (28 January 1864, Halifax (http://www.answers.com/topic/halifax-west-yorkshire) Yorkshire (http://www.answers.com/topic/west-riding-of-yorkshire), England (http://www.answers.com/topic/england) - 19 February 1927, Halifax (http://www.answers.com/topic/halifax-west-yorkshire))
So, a neighbour of YC's, no less.

[embee]

There's a strong trend in "downsizing" in car engines, sufficient output coming from pressure charging, the small VAG engine and the Nissan supercharged engine for example. Package size, weight, parts count etc., running an engine at optimum load/speed for more of the time ("duty cycle").

At last one bike manufacturer has caught on and produced what I've been waiting for. It won't fit everyone else's idea of what they want, but here goes.
Honda NC700 series.

They have reproduced the well developed Honda Jazz combustion system into a bike (right down to bore and stroke), and the BMEP curve is as near as damn it the same. It's been done entirely in order to offer the capability of decent fuel economy. Some bike journalists haven't quite "got it" as one reviewer (http://www.ashonbikes.com/content/honda-nc700x-0) said, they complain that they run into the rev limiter all the time. They miss the point. When driving a car with fuel economy in mind you don't hang it at the rev limiter all the time. It's early days but the numbers coming up on the Fuelly site seem to indicate something like 20% better economy that similar bikes (like the 700 Deauville). It's not a sports bike, but if you want economy it's the way to go.

[Sid Squid]

We were talking about that Honda in Soho recently, to 'get' it you have to stop thinking about it as a 700cc bike that 'only' makes 50hp, and start thinking of it as a 115mph bike that's capable of 80mpg, and has a big whack of easy-to-ride torque low down the rev range.

This, I think, is an interesting development in the world of motorbikes.

[Bibio]

you can make the engine as efficient as you like but until you have a drive train just as efficient than your pizzing against the wind.

for instance i recently serviced my callipers as they were getting a bit sticky and have gained 5pmg.

[yorkie_chris]

We were talking about that Honda in Soho recently, to 'get' it you have to stop thinking about it as a 700cc bike that 'only' makes 50hp, and start thinking of it as a 115mph bike that's capable of 80mpg, and has a big whack of easy-to-ride torque low down the rev range.

This, I think, is an interesting development in the world of motorbikes.

I agree.

Hopefully it's been developed to have plenty useable power right around the 70-100mph range though.

The holy grail to my mind will be when they make something which will do 80mpg at a sensible cruising speed, but still have lots of poke for being silly when you fancy.

[embee]

In the linked article there is this graph comparing the NC700 and a CBF600. The studies they did showed a high percentage of riders using less than 6500rpm for a very large percentage of time, so under liek for like conditions the NC will have "plenty" of performance available, it just runs out if you try to wind it on. But that's the point of the range, if you want high performance you choose a CBF or similar.
http://www.ashonbikes.com/sites/default/files/ash_images/articles/111125/Honda_NC700X_29.jpg

[Sid Squid]

Ooer missus - nice fat bottom end.

And the middle.

And the top, (such as it is), too.

[embee]

I'm sorely tempted to get a test ride of one.

Be interesting to see how low rpm it will actually tolerate from the transmission side, they must have designed/developed it specifically for that so how good a job have they done?

It sounds quite grunty from the youtube clips I've seen, and the auto versions don't waste much time getting up to speed.

[DJFridge]

I've not tested one but I sat on one (in the otherwhere mentioned Honda stand at Surrey County Show) and it all felt quite well put together ergonomically. Most of the reviews I've read seem to be pleasantly surprised by how motorbikey it is to ride once you get past the lack of rev range compared with a traditional motorbike engine. I don't know about the flappy paddle gearbox though (well, flappy button actually). I guess I'd probably get used to it.

[yorkie_chris]

I'd rather have a normal gearbox given there is nowt wrong with them and less to go wrong.

[Sid Squid]

Be interesting to see how low rpm it will actually tolerate from the transmission side, they must have designed/developed it specifically for that so how good a job have they done?
Less primary and more final reduction perhaps - keep the tranny speed higher?

[embee]

Just for interest, the Engine Technology International 2012 awards have just been announced.
http://viewer.zmags.com/publication/f0ecda39#/f0ecda39/1

The new Ford 3cyl 1.0L turbo seems to be a hit with the judges, it'll be interesting to see how it fares in the real world.