Request for a teach-in on piston engine performance

[Guest Breadvan72]

I know that we have the stupid questions thread, but that seems more apt for "my car is doing X, is this because of Y?" questions. I would like to ask those of you who really understand piston engines for some more general info on how and why different engines produce different things.

 

I have a fair basic understanding of how piston engines are put together and how they work.  I know the basics of forced induction systems.  I understand what BHP and torque are.  What I am not so sure about is how various permutations of displacement, cylinder number and layout, fuel supply systems, cam designs, valve systems, and so forth influence performance,

 

My musings have been prompted by recent driving of some peppy DOHC two litre four pot units, an old fashioned 2.25 Landy petrol engine, and  a 2.5 24 valve V6.  I sometimes get to sit behind a lazy large displacement pushrod V8.  I used to fly light aeroplanes, and when I did that was used to large displacement, low revving, relatively low power horizontally opposed four pots (very old skool, with magnetos, mixture controls, and carb heaters). 

 

I have been particularly struck by the differences between the 2 litre DOHC units and the V6.  The V6 has only another 500 cc of displacement, but is waaaaaaay more powerful and responsive than the four pots.  

 

If anyone is interested in discussing this, could I please suggest that we start with normally aspirated petrol engines, but can drift on to forced induction, diesel and so forth later.  

 

Thanks for your thoughts, innit. 

[gordonbennet]

I'm no professional mechanic, though i've stripped and rebuilt and generally tinkered with quite a few engines.

 

I'm generally drawn to larger long stroke cylinder capacities for my driving, as against more cylinders for the same capacity, i have driven four engines for many miles which stand out as having the low speed torque that made my type of driving an absolute pleasure, the three cars i owned, the lorry i was paid to drive.

 

1.   FD Ventora as fitted with the Cresta 3.3 straight six, would pull from 10mph in overdrive right up to 110 without the slightest hiccup, one of my all time favourite cars.

 

2.  5.7 V8 Mustang of 1972 vintage,  bloody effortless and aural ectasy, 3 speed auto which was all it needed such was the torque, she'd do 65 in first, unservoed drum brakes all round made stopping it from 3 figures interesting.

 

3.  1980's Cummins 14 litre 6 pot Diesel, 14 litre big cam with turbocharger, only rated @320 hp but put out a massive 1100+lb ft of torque @ about 1000 rpm, but IIRC produced 90% of that as low as 800 rpm, despite its headline 320hp figure being relatively low compared to the Swedish V8's it was easily a match for them on the road, never driven a lorry since that as competent for general road use, effortless power just listening to the turbo whistling very quiet engine and keeping it between 800 and 1100 rpm all the time made for fantastic economy (every bit as good as the computersied strangled current crap), 1100 rpm gave 70mph all day long cruise, she would run up to 1900 rpm if you wanted to, work the max speed out for yourselves, no limiter either.

 

4.  Toyotas mechanically pumped 3 litre 4 pot turboDiesel as fitted to last issue 70 series Landcruiser and early 90 series Colorado/Prado, this engine proved to be almost impossible to stall, and would pull heavily from stall revs without a murmer without ever showing a puff of black smoke, and the MOT geezer always commented on the lack of emissions, basic engine still made but now in D4D common rail form, i had one in a 57 plate Hilux but still prefer the way the old mechanically pumped lump drove.

 

All these engines shared large cylicner swept volume, smallest was the 550cc Ventora, 710ish Mustang, 750 Toyota and 2333cc for the Cummins.

 

As the old saying goes you can't replace cc, but i'd suggest that indivdiual cylinder cc is what makes for easy driving, and whilst multi valve might be the most efficient on paper again they don't excell at low rpm unless blown in my experience.

 

Engines under 500 cc per cyl i find frustrating, needing to be driven in a relatively small rev band, or as is more likely the result, thrashed mercilessly to get them to move and often prone to stalling, and that gives me no pleaure whatsoever, though others might prefer this type of driving.

 

The only exception i've found to this is Toyotas (again) little 3 pot 1 litre jobbie as fitted to Aygo/C1/107, this little engine for its 300odd cc per cyl bucks the trend, when i loaded these i found you could run them right down to stall revs (much kinder to the buyers clutch) and they must be fitted with a fuel enriching device as they would just keep going and refused to stall...where the most frustrating engine was little 4 pot 1.2 thingies in Fiats 500/Panda, if i had a quid for every one of those buggers i've stalled i'd be a rich man, wouldn't give you a thankyou for one.

 

Don't know if this was what you wanted, apologies for the waffle if not.

[Guest Breadvan72]

That's the sort of thing, many thanks.  I am trying to get my head around bore and stroke and such like stuff.

[Station]

8 cylinders are smoother due to the balance on the crankshaft having - compare this to a three cylinder with three weighted points on the crankshaft and you can understand why they are so thrashy and unrefined. TLDR - the more 'branches' on the crankshaft, the more balanced it is.

You have to consider weights of pistons and size of cylinders, also weight of flywheels come into play. It's really all about physics, boxer engines (as in Imprezas and Porsches) are so good because the piston is on it's side and moves across instead of up and down - there is no weight to lift, only push and pull.

[Guest Breadvan72]

Flywheels, yes - this is the sort of Dark Arts stuff wherof I wotteth not of.

 

Is there a good website or book on this?  Not Noddy "how does an engine work?" stuff, as even I have a vague concept of that, but more a discussion of the various factors that produce various characteristics of engines.  

[Asimo]

Have a look around the Autosport technical forum. Although there are a few dumb and/or opiniated posters, some of the posts are genuinely educative and often link to really useful sites.

 

http://forums.autosport.com/forum/8-the-technical-forum/

 

I posted a question on here and part of the reply was a link to this site  http://www.pattakon.com/

 

Lots of useful stuff on there, particularly about engine balance. The guy is obviously an academic engineer and if you ignore some of the stuff about new, possible configurations for engines, there is a lot about our sort of everyday car engine.

[Danblez]

8 cylinders are smoother due to the balance on the crankshaft having - compare this to a three cylinder with three weighted points on the crankshaft and you can understand why they are so thrashy and unrefined. TLDR - the more 'branches' on the crankshaft, the more balanced it is.

You have to consider weights of pistons and size of cylinders, also weight of flywheels come into play. It's really all about physics, boxer engines (as in Imprezas and Porsches) are so good because the piston is on it's side and moves across instead of up and down - there is no weight to lift, only push and pull.

 

Hmmm never really thought of this but do they suffer more wear on the side of the cylinder which is at the bottom?

[castros_bro]

Yer Hairyplane prop tip max speed about 0.9 x speed of sound (temp dependant).  So doing sum maths with the prop diameter, any reduction gearing gets back to max engine speed. As you don't need to do slow three point turns whilst parking at Lidl * you only need three throttle positions/ power settings,  going up, going horizontal, going down which are not the characteristics for yer average road vehicle.  I have read and reread Tow Stroke Tuning by A. Bell and can recommend but have yet to read his four stroke one.

 

 

 

 

 

* other supermarket carparks are available for not parking planes in.

 

Edit: have just bought the Four Stroke on from and online book webshite

[barefoot]

I'm going to go against the flow here and say that my 944S 16v, has a cylinder capacity of 620cc, but it must have a flywheel the size of a button because its easy to stall. It doesn't pull well below 1800rpm and it's 4,000 before it really starts to sing.

My old Scirocco is just 398cc each and will pull a 25/1000 top gear uphill at 1200 rpm

These are down to cam profile as far as I know, but why?

[Guest Breadvan72]

That is precisely the sort of info I am interested in.   All of the factors, how do they work together?  Bore, stroke, valves, cams, fuelling, etc, etc.

 

BTW, aero piston engines have throttle quadrants as they are operated at more than three settings.  You set the throttle according to what you are doing.  In a fixed pitch prop machine, you almost never take your hand off the throttle lever.  For example, are you cruising for max endurance?  Are you making a steep turn (add power)?  Are you landing without flaps because of gusty winds? Are you doing dodgy low level aerobatics over your mum's house inside the Heathrow Control zone?   (CAA say: "come for tea, no biscuits".  RAF say:  "my office, your hat").

 

You also faff about with the mixture according to altitude and air conditions.  

 

Throw in a constant speed (aka wobbly aka variable pitch) prop and you get to faff about with manifold pressure.  Most of the kites have no reduction gears, so WYSIWYG with engine revs, the propeller turning with the crank.

[Formula Autos]

Multi-cylinder engines benefit from a reduction in reciprocating mass - which basically means less weight for the crankshaft to have to 'throw' back up the cylinders. This makes it easier for the engine to rotate, and thus it can help it rev faster. That's why Ferrari persist with V12s.

 

Having said all that, performance isn't the only issue - multi-cylinder engines are inherently more balanced (more, lighter, points of contact with the crank for the given cc of the engine), and thus smoother*. The V12s in Jaguars being more geared towards this end of the spectrum than Ferrari V12s, for instance.

 

More cylinders also equal more chances to accurately control fuel, air, and exhaust - albeit usually by lobbing more fuel and air in, which also has benefits for performance. Though not necessarily fuel economy, obviously.

 

With all that in mind (and others' comments about the benefits of Boxer engines), probably the best engine for a motorsport application would be a Flat-12. Provided it can be made to fit sensibly, without buggering up chassis dynamics. Though that means straying far from Autoshite territory, so I'll leave it there.

 

All of the above has been gleaned from reading Setright et al over the years, and so may have been turned, by my brain, into utter wank. If anyone can pick holes in the above, feel free.

 

*Generally, if the number of cylinders is divisible by four - to tie in with the four stroke cycle, I think. Hence the minimal smoothness benefits of a 5 pot or V6 in relation to a four banger of the same cc, compared to a V8 or V12. Or somesuch.

[Rocket88]

Always prefered high capapcity, relatively low powered engines to tiny screamers. Big cacacity v8 lumps are wonderful, especially when a blip of the throtle makes the chassis move in sympathy.......

[alf892]

Long stroke/small bore tends to give a nice lazy power delivery. Each explosion produces a nice long push as opposed the short sharp jab of a more square engine. So once the get the short jab engine spinning it will produce more power but it will still lack the push (torque) of it's more leisurely brother

 

Mutlivalvers tend to need to rev to come into their own. Forced induction helps I believe but apparently its physics.......I don't know what specific bit though!

[320touring]

If an engine is 'oversquare' (that is, the stroke is shorter than the bore) it tends towards being easy to rev, and faster to rev-e.g. bike engines, my beloved m20b20 etc.

 

This means that the power and torque bands are further up the rev range than on long stroke engines. This makes them buzzy to drive, unlike a big capacity long stroke motor.

 

Obviously these characteristics can be tempered by camshaft and flywheel setupsne.g. heavy flywheel makes it slower to rev, small duration camshaft moves torque band down the range.

 

Geneqlly, the straight 6 hs the smallest config that is harmonically balanced, hence they are much smoother than a 4. But a 4 cyl of same capacity will generate more torque than a 6 as bore and stroke per cyl are larger.

[red5]

What you want is a bloke what teaches Automotive engineering......oh, wait....

[Guest Breadvan72]

We need yer lecture notes, bloke.

 

I talk to my dad about these things, but he is knocking on a bit now, and isn't as clear as he used to be on this stuff.  In any event, his car industry professional world was more about manufacturing processes than about the specifics of the engines.   He was what used to be called an Industrial engineer - do they still call them that now?  

[Junkman]

To really understand the design principles of an internal combustion engine would indeed require several years of study, hence it is taught at universities.

An interesting fact is that the principles are known since the 1920s, including the volumetric efficiency and gas flow calculations. So much for innovation.

 

The first thing required to understand is that a four-stroke engine working to the Otto principle has one power stroke only every 720 degrees of crankshaft rotation.

Hence multiplying the cylinder count decreases the power stroke intervals per 720 degrees of crankshaft rotation by 720 degrees divided by the number of cylinders.

Consequently, in layman's terms, the more cylinders you have, the smoother the engine is running.

However, space and many other constraints, one of the other big ones being production cost, require specific arrangements of the multiple cylinders relative to the crankshaft,

each one having its own advantages and disadvantages. The most common ones, like I4, I6, V6, V8 and V12 are used, because experience has shown that they provide

the best balance between advantages and disadvantages.

[Guest Breadvan72]

Cheers, all.  

 

OK , try this dumb-ass question.  Answers in crayon, please.

 

What do we mean when we say that a car has an aggressive cam?

 

Pub example:  "My dad's Sunbeam Rapier is boggo, and it's a right old knacker. That bloke at the classic car show had a H120, and he said that the Holbay cam is more aggressive, so his car goes like stink".

[Junkman]

The lobe profiles of the camshaft are ground in a way that opens the valves for a longer duration and lift, hence allowing for a better gas flow.

The disadvantage is usually a lumpy tick over and less torque in certain rev bands.

[jonny69]

I'll stick something in about cam grinds then. In short: low lift and short duration means low-down torque and efficiency; high lift and long duration makes higher power at higher revs at the expense of low-down torque and efficiency.

 

Increasing the cam lift means the valve opens further and the engine can suck more air/fuel in and push it out easier, so to a point it develops more power, more torque and it can increase the efficiency. There is a maximum lift you can take a valve to and above that maximum it will not flow any more. It is mainly related to the diameter of the valve.

 

Torque is how much twisting power the engine has. Horsepower is the rate at which it generates torque, so the higher the revs at a given torque figure the higher the horsepower. To build more horsepower an engine needs more torque and more revs. To get more revs, a cam needs more duration, that is the valves need to be open for longer. They need to open earlier and close later so that the engine has its best chance of getting every last bit of air/fuel in and out. At very high revs, the gases are moving through the engine at very high speed and the inlet and exhaust valves can be open at the same time. How long they are both open for at the same time is the amount of overlap. As a general rule, the higher revving the cam grind is designed for, the more overlap it has. The more duration and overlap you have to build high revs, the more power you sacrifice at the bottom end because the valves are not fully closed together. On very high overlap grinds, the engine will push a certain amount of the combustion gases back into the inlet at low revs, hence race engines run rough at low revs.

 

So big lazy torquey engines have mild cam grinds with not too much lift and short duration. Revvy 4-pots have higher lift and longer duration.

[MrDuke]

things about cams and valves

 

Which is why variable-timing systems can be so useful, giving decent torque at low revs, and another kick at high revs. Plenty of people think Vtecs are overrated chav-fodder, but IME they make really good everyday engines.

[Peter Patina]

Hmmm never really thought of this but do they suffer more wear on the side of the cylinder which is at the bottom?

 

I was told by our fitter that Leyland 680s as fitted horizontally to Leopards did used to ovalise their bores eventually, wheras the same engine in vertical config in an Atlantean didn't.

[Inspector Morose]

Without trying to teach anyone to suck eggs, this book is quite good at giving one an insight on mechanical tuning principles. It might say A series but a lot of the information can be transferred to other types of lump.

 

[Mr_Bo11ox]

^^^^ strongly agree with that, its an excellent book for understanding engines in general.

[Guest Breadvan72]

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.

[gordonbennet]

As for books, one of the best was the 70's printed AA Book of the Car, a large volume and obviously well out of date now with everything being electronic and computerised, but the basics are as valid now as they were then, not just for engines but fuel cooling brakes and suspension systems.

I see they are still available cheaply, used of course, on Amazon and the bay of E.

 

I think an up to date version of this book would be gold now for younger car enthusiasts, who by and large don't have the luxury (in knee deep scrapyard shite) of learning by trial and error and scrapyard parts supplies for their cars, the way some of us old farts did.

[Guest Breadvan72]

I like the Virgin Car Book, which is funny in an Autoshitey sort of way, as well as being informative.    "In your constant war with the automobile, its agents, minions, and familiars, this book is on your side".

[HMC]

As for books, one of the best was the 70's printed AA Book of the Car, a large volume and obviously well out of date now with everything being electronic and computerised, but the basics are as valid now as they were then, not just for engines but fuel cooling brakes and suspension systems.

I see they are still available cheaply, used of course, on Amazon and the bay of E.

 

I think an up to date version of this book would be gold now for younger car enthusiasts, who by and large don't have the luxury (in knee deep scrapyard shite) of learning by trial and error and scrapyard parts supplies for their cars, the way some of us old farts did.

 

I dug this out of our loft when I was 8 or thereabouts and Ive never been far from it since. Its sat gathering dust on a shelf next to me as I type. Good technical sections.

 

Theres quite a big chapter on 'improving the older car' stuff thats really of its time. Step by step pics of stuffing chickenwire and filler into the sills of a MK2 jags and the like, as endorsed by the AA! 

 

[willswitchengage]

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?  

 

Common Rail injectors are fed by a common high pressure fuel source (the "common rail"), and each injector is controlled by the ECU to deliver the precise amount of fuel, over a precise period, at the precise time - decisions being made from the inputs of various sensors detecting temperature, load, EGR status etc. It's an "intelligent" system unlike a mechanical fuel pump, which simply meters the fuel and sometimes has a pressure-opened valve to increase the metering if turbo-boost is present.

 

 

The most common ones, like I4, I6, V6, V8 and V12 are used, because experience has shown that they provide

the best balance between advantages and disadvantages.

 

Straight six is the simplest fully balanced engine, as are V12s and multiples. Three pots, straight fours, fives and V8s are not balanced, and will all need balancing shafts. Increasing the number of cylinders doesn't make a difference... technically the smallest fully balanced engine is the 2CV/BMW boxer - flat two. V engines are horribly unblanaced and problematic, they're used purely for packaging reasons (they take up little space).

 

I6, V12, F6 and boxer are balanced. Nothing else (conventional) is.

 

 

All of the above has been gleaned from reading Setright et al over the years, and so may have been turned, by my brain, into utter wank. If anyone can pick holes in the above, feel free.

 

*Generally, if the number of cylinders is divisible by four - to tie in with the four stroke cycle, I think. Hence the minimal smoothness benefits of a 5 pot or V6 in relation to a four banger of the same cc, compared to a V8 or V12. Or somesuch.

 

Incidentally why you shouldn't believe anything motoring journalists say!

[Luckythirteen]

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...