Request for a teach-in on piston engine performance

[jonny69]

I'd love to take a superbike engine apart and see it inside. It must be a super short stroke with a big bore, i.e. way oversquare. If you look at a Fireblade piston and compare it to a piston out a 1 litre inline 4 car engine, you'll see that there's almost nothing of the Fireblade piston in comparison. It's super short and there's practically no skirt on it. The flywheel is the same, it's tiny and so it the clutch. There is also practically nothing of the crank and the Fireblade one looks heavily counterbalanced. So everything inside the engine is minimal and lightweight and designed so it can spin freely with little effort. If you then look at the heads, they typically have 4 valves per cylinder, big ports and a throttle body or carb per inlet port which is rare on a car.

 

The curious thing about bike engines is that the cam grinds are usually quite mild. You'd think that an engine that is designed to rev to 11-or-12000rpm would have quite a wild cam grind - but they don't. I'd like someone to explain that one!

[Stanky]

I found this a treasure trove of free info on all manner of car related info:

 

http://www.carbibles.com/

 

It also has VERY CLEAR PICTURES for those of us who are too stupid to understand the complicated words. Many happy hours have been spent finding out information which I will never use, but which is interesting nonetheless.

 

Edit - its is quite American in areas though

[Guest Breadvan72]

Pop the hood, buddy.  Let's see whaddya got.

[Guest Breadvan72]

Big thankings to all who have contributed to the search for enlightenment.  I may be none the wiser, but I am much better informed.    I encourage other iggerant types like me to chip in with general questions about scayree Voodoo engine magick, but we should try not to make this Stupid Questions Amnesty the Second.  

[CreepingJesus]

Ultimate minimalism in the bike engines must've been the 250cc fours of the mid-90's. 20,000rpm on tap, but they were gutless under 10 apparently!

The cam profiles on bike engines aren't too aggressive (either in lift/duration or on the parabolic curve of the lobe face) to avoid flat spots. The downside of their lightness is sensitivity to vacuum depletion causing flat spots and rough running. Ask anyone who's fitted a cheapo 'performance' can or filter how that feels!

Those 250s used sprung valves: MotoGP bikes used F1 style pneumatic valves to do the same, for a while.

But there's also Ducati's 'Desmodromic' system. No valve springs - it uses an auxiliary cam to positively shut the valves. Invented by Merc in the 1920s, pilfered by Fabio Taglioni in the 70s!

[MrDuke]

One of the things that's always fascinated me is the black art of optimising airflow, either in or out of the engine. Common sense would suggest that the whole air path should be as straight and unrestricted as possible, with all surfaces polished smooth and no protruding edges, but apparently that would produce woefully little power (nice noises though). I can understand the need for exhaust back-pressure in small engines, but the whole business of creating turbulence and circulating air currents on the way in leaves me befuddled.

On the subject of Vizard, I used to have his Theory & Practise Of Cylinder Head Modification, which despite several valiant attempts, I could never quite understand.

[CreepingJesus]

The old Panther 600cc singles were a good example of that. Very long stroke = slow moving piston: the relatively small carb meant that the cylinder vacuum pulled what it needed, progressively. What with the very mild cam, and low compression, it made for a torquey, slow revving engine, which was forgiving of crap 2 star petrol.

Starting one was another matter: you had to kick it over compression, and if it kicked back, you were for the high jump. And maybe a broken leg.

[PhilA]

We have an old Hyster fork lift at work. It has an elderly Ford inline six from their "spectacularly detuned truck engine" range.

 

It has a very large flywheel, low compression and has a very mild cam. It has trouble revving past what sounds like about 3000 RPM, but is all about the torque.

 

It'll idle smoothly at about 400 RPM with a sound that I can only really explain as "slapping egg-cups with a wad of Post-It notes.

 

Prlpl-plrlplpl-plpft-plplrrplpl

 

Compared to my car, which has a similar displacement in V8 format.. idle is 800 RPM, torque begins coming in at 4000 all the way through to the red at 6000.

Variable valve timing, tuned porting, computer controlled ignition, twin spark plugs.. the two engines are almost a world apart.

 

Phil

[Asimo]

Balance and engine configurations

 

The flat twin or boxer twin (2CV etc) is not perfectly balanced: the primary and the secondary forces are balanced but there is a significant couple that causes the engine to twist about the centre of the crankshaft (when viewed from above). Very obvious when sitting on a BMW boxer at tickover. This occurs because the opposing pistons are not actually opposite. 

It is the same case with bicycle pedals: they can be statically balanced but as soon as the pedal / crank assembly is spun, the bicycle shakes.

 

The common or garden V8 is interesting.

This configuration is fundamentally out of balance in that the forces due to rotating and reciprocating parts do not cancel. However, these forces produce an out of balance force that is rotating in the same direction and at the same speed as the crankshaft. This out of balance force can be completely cancelled with an appropriate weight at each end of the crank assembly. Some engines fit this weight on the crankshaft as an extension to the crank webs at each end, other designs fit the weights to the crankshaft pulley and flywheel. Either way, the result is a smooth,  engine.

[jbz2079]

The old Panther 600cc singles were a good example of that. Very long stroke = slow moving piston: the relatively small carb meant that the cylinder vacuum pulled what it needed, progressively. What with the very mild cam, and low compression, it made for a torquey, slow revving engine, which was forgiving of crap 2 star petrol.

Starting one was another matter: you had to kick it over compression, and if it kicked back, you were for the high jump. And maybe a broken leg.

In real terms the piston speeds for long stroke engines is high,just because it has further to travel.

Also big pistons take a lot of stopping at each end of the stroke and when you only have one things tend to join in, Like the whole bike, you and anything close to that slow chuff out the exhaust.

 

British large capacity twins are a good example of over streaching things.

The 650 Triumph twin was like a road drill with both pistons traveling up and down together but it would rev, the 750 version does not rev as easy

and never seems to have the ripping cloth sound from the exhausts.

 

The 750 Commando enging is well known as a time bomb.

Everything at it's limit, as you might expect when the engine was designed as a 500 twin and ended up a 750.

[jbz2079]

Speaking for myself, I am not all that interested in tuning or in racing engines, more in why different types of road car engines do what they do.

 

Take the Lampredi twin cam in two litre form, aspirated by a single twin choke Weber.  Take the same engine and put a smallish twin rotor supercharger on it.  Compare and contrast the Busso V6 in 2.5 litres.    The latter is much more revvy, and much more powerful.    From answers above I can deduce some of the reasons for this, but, being a dullard, I am not seeing the complete picture yet. 

 

Also, changing the subject quite a bit, can someone please explain why a common rail diesel engine is a cool thing, if it is a cool thing?  

 

Sorry for the all the questions, but I find this stuff fascinating, simply because I like cars (I must be a dickhead) and would like to understand them better.  I have no intention of  doing any tuning, racing, or other engine work.

Common rail diesel injection is a good thing because the injectors are fired from the computer.

This means the fueling can be shap and precise and can be tailored to what boost pressure the turbo is making.

This means the fuel should be better used with less unburnt fuel leaving the engine (emissions).

My 406 HDI 90 was generally 10 mpg up on my old XUD TD Xantia but the old XUD was better to drive with as it was unfussy and as long as it had turbo boost was tourqey.

The HDI told you how it wanted driven, to the point that deliberatly pooting around trying to save fuel was a waisted excercise.

It needed to be driven in the high boost part of the engines range to be economical.

 

The can also do tricks like several injection squirts on each power stroke to make the engine run quieter and burn the fuel more completely.

It might also be cheaper to make a single high pressure pump with electric injectors than a high precission mechanical fuel pump.

[mat_the_cat]

Ultimate minimalism in the bike engines must've been the 250cc fours of the mid-90's. 20,000rpm on tap, but they were gutless under 10 apparently!

 

How about the 125 cc fives, or the 250 cc sixes of the 60s? Crazy stuff!

"had to be kept between 21,000 rpm and 22,000 rpm to be at its best"

http://www.motorcycle-usa.com/289/15555/Motorcycle-Article/Memorable-Motorcycle--Honda-RC148.aspx

 

The common or garden V8 is interesting.

This configuration is fundamentally out of balance in that the forces due to rotating and reciprocating parts do not cancel. However, these forces produce an out of balance force that is rotating in the same direction and at the same speed as the crankshaft. This out of balance force can be completely cancelled with an appropriate weight at each end of the crank assembly. Some engines fit this weight on the crankshaft as an extension to the crank webs at each end, other designs fit the weights to the crankshaft pulley and flywheel. Either way, the result is a smooth,  engine.

 

And that is why a V8 is the engine of WIN! Here's a question - why does a V8 sound better than a four pot? Is it all to do with a perception of performance?

[jbz2079]

I'd love to take a superbike engine apart and see it inside. It must be a super short stroke with a big bore, i.e. way oversquare. If you look at a Fireblade piston and compare it to a piston out a 1 litre inline 4 car engine, you'll see that there's almost nothing of the Fireblade piston in comparison. It's super short and there's practically no skirt on it. The flywheel is the same, it's tiny and so it the clutch. There is also practically nothing of the crank and the Fireblade one looks heavily counterbalanced. So everything inside the engine is minimal and lightweight and designed so it can spin freely with little effort. If you then look at the heads, they typically have 4 valves per cylinder, big ports and a throttle body or carb per inlet port which is rare on a car.

 

The curious thing about bike engines is that the cam grinds are usually quite mild. You'd think that an engine that is designed to rev to 11-or-12000rpm would have quite a wild cam grind - but they don't. I'd like someone to explain that one!

Easy, it does not have over a metric tonne of car to move from a standstill and will not last 15000 miles when asked to do so.

There are several Fireblade and Hyabusa engines in old BL Minis, I'd love to see one pull off the line with four people inside.

Also the fuel consumption will probably be horrendous.

[Dave_Q]

To expand on the diesel engine question, the reason for manufacturers changing from rotary pump to pd to common rail technology is for emissions reasons.

 

Basically in a diesel engine the window in which the fuel is injected is directly related to the pressure it is injected at (higher pressure = more fuel in more quickly.)

 

Any fuel injected after TDC will not burn fully and make the bad emissions.

 

Injection pressures have gone from around 200bar in rotary pump systems, to 2-4000 in a PD system, to 20,000bar or more in a modern common rail (20,000 times atmospheric pressure!)

 

This enables more of the fuel to be injected in the optimum window and enables things like multiple injection events as mentioned before.

 

However, is it a good thing in itself? Probably not. Extra complexity means infinite repair bills.

[Luckythirteen]

So, why is it that diesels run cold, then? Or at least take forever to warm up?

[Asimo]

why does a V8 sound better than a four pot? Is it all to do with aperception of performance?

 

Most people like the throbby, burbley V8 sound (not me though). This sound is due to the very uneven firing order of each bank of four cylinders. The overall firing order of the V8 is even, there is a bang every 90 degrees of crankshaft rotation.

 

However, the firing of one four cylinder bank of the V8 is

 

Bang-180deg-Bang-90deg-Bang-180deg-Bang-270deg (etc etc)

 

Obviously the exhaust system is at least in part separate for each bank of the engine so the sounds from each half of the exhaust have the uneven throb of the firing order. IF there were eight equal length and identical pipes from exhaust port to tale pipe, all the throb would be gone from the exhaust sound.  

 

A lot of the sound you get from a four cylinder engine is actually the effect of the engine vibration upon the rest of the car. The inline four produces a vertical shaking vibration at twice the engine speed which makes much else buzz and this is responsible for some of the coarse sound of the four.

 

And yes, much is down to perception, how we like different sensations as well as how loud, even and what frequency sounds are.

[jbz2079]

Personally I like the sound of a straight six, Straight sixes tend to run smooth but the do give their crankshafts a real hard time.

Because of the crank length and the six con-rods pushing on it tends to set up a twisting motion that looks a bit like a dog shaking water off it 'self.

 

They are balanced in the build and usually wear a front damper that is designed to take the twisting energy out the crankshaft.

On sixes if the damper is worn or damaged the crankshaft can fracture. 

[Guest Breadvan72]

Thanks for all the continued teachiness.  I am loving it, and even vaguely understand some of it, but I ain't doing no test!

[Dave_Q]

So, why is it that diesels run cold, then? Or at least take forever to warm up?

 

Diesels have better thermal efficiency because of their higher compression ratio. More of the fuel/air mixture energy is converted into explodeyness, therefore less is wasted as heat, therefore longer to warm up.

 

I have the proof of why this is so written down somewhere, but it involves lots of maths. 

 

Nobody wants that.

[jbz2079]

`That is about spot on Dink, Diesel fuel has more emery stored in it per CC than petrol, it just takes a better engine

to extract it.

Also diesel engines are heavier built with heavier cast blocks to withstand much higher compression and so have thicker pistons that weigh more.

All this metal takes far longer to warm up, that's why the BMW powered diesel Rover 75 had a auxiliary water heater plumbed into the engine cooling circuit, to speed up warm ups from cold starts and make the interior heater warmer.

 

The original Ford 2.5 DI in the Transit was a typical example of not getting hot enough in really cold weather, they always struggled to run heaters in minibuses.

[Stanky]

Interestingly* butter has more energy stored in it than TNT, but its not able to be released in a rapid fashion (unlike TNT) and therefore is GR3 for IEDs

[jonny69]

One of the things that's always fascinated me is the black art of optimising airflow, either in or out of the engine. Common sense would suggest that the whole air path should be as straight and unrestricted as possible, with all surfaces polished smooth and no protruding edges, but apparently that would produce woefully little power (nice noises though). I can understand the need for exhaust back-pressure in small engines, but the whole business of creating turbulence and circulating air currents on the way in leaves me befuddled.

On the subject of Vizard, I used to have his Theory & Practise Of Cylinder Head Modification, which despite several valiant attempts, I could never quite understand.

Theory on flow changes round so often it's difficult to keep a track of what's current thinking. I'm firmly in the straight and unrestricted camp but it's worth bearing a few things in mind-

 

A polished port flows best with dry fluid going down it. If you have air AND fuel moving down a port, if the surface is polished it tends to promote the fuel beading on the surface which disturbs the flow and breaks up the atomisation of the fuel. On a carburettor engine it's better to not polish the ports, but on a fuel injected engine where the fuel is not travelling with the air, it's better to polish.

 

Straight route in with no sharp bits, restrictions or steps means less resistance and better flow. End of. But there is the subject of swirl. Swirling up the mixture helps dissipate the fuel and in turn makes a more efficient burn, which in turn means more power from the same amount of fuel/air. Engines have had lots of ways to induce swirl and squish from cleverly angled ports to shaped combustion chambers.

 

It really depends on the design of the engine and even then there are often many sides to the theory. Take old side-valve engines, which I've tuned a few. I always went for as much compression as I could (to make best use of modern petrol), as much lift as I could get away with (to get as much mixture in as I could) and chamfer the edge of the bore on the valve side to help the mixture slide in. It worked great for me. The other theory is to run lower compression so the engine can spin more freely with less resistance, less lift so that there is a lot of space above the valve so the mixture can flow both under and over the valve, and some chamfer and some don't. It works great for them, but they're convinced I'm doing it wrong. I'm convinced they're doing it wrong! So it's worth remembering that many engines are a bit of a compromise and there is often more than one way to build power.

[PhilA]

Then you get the sink-plughole-theory engine (Chevrolet Vortec) with a rather indirect path for the intakes:

 

 

--Phil

[Luckythirteen]

Diesels have better thermal efficiency because of their higher compression ratio. More of the fuel/air mixture energy is converted into explodeyness, therefore less is wasted as heat, therefore longer to warm up.

 

 

So does that tie into why common rail is better? As in, if you chuck more fuel in, the temp stays under control more easily? IYSWIM.

[red5]

Common rail (injection) is more to do with very precise metering of fuel (which can lead to more powah/moar ecocococnommy etc) Oddly, after SI engines adopted mechanical fuel injection from CI engines, the next 'leap' forward in CI engine technology was the adoption of SI style common fuel feed layout and electronic 'injection' at very high (cut steel/hands etc) pressures.

 

Whoops.

 

SI  - spark ignition

CI  - compression ignition

 

Common rail - expensive, lethal, expensive..

 

Christ, my students must hate me...

[forddeliveryboy]

The old Panther 600cc singles were a good example of that. Very long stroke = slow moving piston: the relatively small carb meant that the cylinder vacuum pulled what it needed, progressively. What with the very mild cam, and low compression, it made for a torquey, slow revving engine, which was forgiving of crap 2 star petrol.

Starting one was another matter: you had to kick it over compression, and if it kicked back, you were for the high jump. And maybe a broken leg.

 

Long stroke = higher piston speed and more rapid piston acceleration, less friction, less piston mass, time for a longer burn (ideal for veg oil)

 

So, why is it that diesels run cold, then? Or at least take forever to warm up?

 

It's the direct injection diesels which warm up slowly because they're so efficient and waste less heat. Older diesels generally warmed up fast.

 

 

Nobody's really mentioned personal preferences much - me? I love boxers, as fitted to bikes - Douglas, Brough, BMW, Honda and many others;  cars - Subaru, Tatra, Porsche, Citroen, Alfa-Romeo, Ferrari and more;  plus many, many light aircraft. There are only two downsides - the vehicle has to have an engine bay wide enough with access for servicing and they're more costly to make, which is why there aren't many more cars with them.

 

They're stronger, like for like (shorter crankshaft), so longer-lived, They are inherently balanced from a reciprocating mass pov so no counterweights or crank webs or extra shafts are required. This means a lighter reciprocating mass, so more responsive and keen to rev, Not having to attach heavy lumps or extra contra-rotating shafts to balance them means they're smoother, and equally smooth throughout the rev range. Not only do the reciprocating masses cancel perfectly, but also the effects of the explosions. The low centre of mass means a car is more stable and will corner better.

 

On a long fast journey, they can be revving at high revs without getting noisier or rougher, just the pitch changes. They shrink a long trip, in my experience. Anyone with any real experience of a 2cv will appreciate how good the engine is (mention of Setright earlier in the thread reminded me of his admiration for the Citroen twin, calling it the best four stroke engine ever made, largely on the grounds of effectiveness and simplicity).

 

This little gem produces just 29hp yet can haul a car with poorish aerodynamics at reasonable speeds and cruise very easily at 65mph, or even flat out at 70, without getting inside the occupants heads. It sounds noisy at first, the further you travel the less it intrudes. I've travelled in some expensive saloons in which the engine has become wearing at less than 70% of top speed.

 

Tuned Italian-style rather than Frenchly, with exhausts and intakes, ports and manifolds all designed to minimise the drag of air through the engine, the sound is gorgeous, my old SudSprint (the little 1300) was a peach of an engine, even if a little cranky and worn at just 110,000 miles. I've driven the French flat four at a sustained 8000rpm mile after mile, with more than three times the mileage. A flat six can be sublime, although I'm not too keen on German engines. The Subaru one is lovely, though.

 

Since we all know what a VW boxer engine sounds like as well as a Subaru and a Porsche and a 2cv, here's a french flat four with a little less sound inhibition than usual.

 

http://www.youtube.com/watch?v=uGNOqhrrar0

 

and its Italian cousin

 

http://www.youtube.com/watch?v=iI7Th4GPYXA

 

 

An interesting page on engine layouts: http://www.mekanizmalar.com/menu_engine.html

[Asimo]

Flat /  horizontally opposed engines

 

They're stronger, like for like (shorter crankshaft), so longer-lived, They are inherently balanced from a reciprocating mass pov so no counterweights or crank webs or extra shafts are required. This means a lighter reciprocating  rotating mass, so more responsive and keen to rev

 

Tatra engines are all V8s I think. 

One of the tragedies of motoring history is that the Citroen DS never got the flat-6 it was designed for.

Another one is that the Lancia Gamma flat-four was made of cheese and hope. But it was lovely when it worked.

 

I've owned 6 flat-four cars from four different makes and briefly driven an Alfasud and the one I would have again like a shot is the 1015cm³ Citroen.  (See http://www.citroenet.org.uk/passenger-cars/michelin/gs/flat-4/engine.html )

 

Time now for a flat-6 perhaps?

 

[MikeKnight]

I've never owned a V engined vehicle, I'd like to experience it though so I have something to gauge against.

 

I've driven my fathers V6 Jag X-Type, but that's in a Jag so it's buttery with a smooth torque curve anyway.

 

I think the "best" engine I've driven in terms of size, number of cylinders and smoothness is the 2L push rod Straight Six in my Triumph GT6.

[dugong]

I r lerning too...

 

Asked dad why the less than a litre engine in his Fireblade does 11,000rpm and 150hp, and the 1.2L engine in my Clio makes a third of that, at half the revs. And is physically bigger. The explanation amounted to 'horses for courses', in that the bike engine is trimmed down to nothing in every way, but the car engine isn't. I'd only asked as I knew someone who was going on about putting a bike engine in a car, and although his eyes lit up at the thought, he pointed out that a bike engined car would be tricky to drive, with so little flywheel. Must be fun though, or people wouldn't be doing it!

Damn you, quest for knowledge! That's more stuff I need to learn!

 

Lern me moar...

A bike engine would be awful in an average car because they're designed for power and not torque. Something like a Hayabusa engine would struggle with the weight of a car unless it was a tube-framed special, and you'd need a seven or eight speed gearbox to keep it in the power band to multiply what torque the engine did have (at stupidly high revs). 

Bike engines are also not designed for slogging at low revs or in traffic, they'd just bog down everywhere. 

 

Bike engines get away with producing sod-all torque because the bikes they're pushing weigh very little to start with, unless you've got a Honda Goldwing or a Laverda Jota. The former can tow small cars and the latter had more torque than the Endura E in my Ka...........that would be a fun engine swap. Small capacity, torquey bike engines would be hilarious fun in a sub 500kg car - that's why I was really impressed with Morgan's new three wheeler. 

[Pete-M]

I quite regularly play with BSB Superbike engines. They get quite good power and torque readings from some of the modern stuff - a basically standard 2014 1.0 bike engine engine produces 200 bhp with 88 lb ft of torque, Which could hurtle a lightweight car around without any real problem, but although the torque curve is pretty flat there's bugger all there below 3500 rpm then it creeps up to 80 lb ft aÃ…Â¥ 6000 ish until the torque peak at 11200 rpm.

 

There's nothing too magical in them. Four small lightweight pistons, DOHC head, eight throttles and a very very clever management system.

 

Small capacity so the only real ways to get more power without forced induction or breaking the rules are to add more revs, blueprint the engine to within an inch of its life and spend days at a time doing dyno setup. A day on the dyno can get 5-10% extra power as those 200 horses a road bike delivers can get a bit shy occasionally.

 

Full spec BSB bike can often produce 215-220 bhp but they need rebuilding every 400 miles or so. Go any further with tuning them and the riders complain about not being able to balance the power delivery as they get a bit sudden.

 

215 bhp from a 1.0 normally aspirated engine is pretty bloody impressive.