i prefer "you cant beat cubic inches"
|
Basically, from an engineer's viewpoint, the advantages of a turbocharger can be summed up as higher power density, which leads to the advantages of lower package bulk/weight, better performance, and improved fuel economy and emissions.
Its not all clear cut - one of the big disadvantges (in addition to cost and complexity) is increased exhaust-side thermal inertia - in a nutshell the exhaust side takes longer to heat up which leads to a poor cold emissions profile. This is a problem when cars are used for many short journeys.
The driving response is also (IMHO) not that great - the link between demand and output is indirect (e.g. turbo lag). Moreover any charged engine has be designed with a low compression ratio. So a typical turbo engine might be 8:1 static CR. This is no problem when the engine is under 1 atm boost (effective CR 16:1), but at low rpm off-boost the engine is operating inefficiently and not developing much power. E.g. a Subaru EJ25T engine develops about 230bhp max., but take the turbo off and the engine probably doesn't hit 100bhp max. You notice this in the lack of low speed pull from junctions etc when a notionally 'less powerfull' NA engine feels a lot crisper.
For these reasons there is a lot of interest in supercharging as a means of increasing charge density - it has many advantages over turbocharging - exhaust thermal interia doesn't change, lighter and cheaper, simpler installation, far better low-speed performance. Quite a few companies are working on traction-drive centrifugal blowers, which look very promising.
|
Is 'traction drive' a device that varies the gearing between crankshaft and blower, Aprilia? Something of the sort is needed to make engine-driven blowers viable for bread-and-butter cars.
My understanding of engine-driven superchargers is that the problem is with the rigid gearing between engine and blower. The blower has to be geared up above crankshaft speed to give good response at low engine speeds, but this means excessive pressure and considerable frictional losses at high engine speeds, as well as fuel consumption much worse than with a turbo. Some manufacturers - Mercedes in the 1920s and 30s? - fitted a clutch so that the blower could be turned off after a few seconds to avoid melting the engine. People liked the screaming noise though.
|
"fitted a clutch so that the blower could be turned off after a few seconds"
I think the idea was that you could turn it ON for a few seconds, when you needed a bit of extra boost! Not a bad idea, really - you could link it to the throttle so it worked like the kick-down in an automatic...
IIRC, the drawback was that because it wasn't in use all the time, it had to be placed in front of the carburretor, i.e. blowing into it rather than drawing mixture through, which made it less efficient. Shouldn't be a problem with injection, though.
Edited by J Bonington Jagworth on 27/10/2007 at 21:47
|
"fitted a clutch so that the blower could be turned off after a few seconds"
He does this in the film Mad Max 2; has a big red button on the gear lever which he uses to disengage the drive to the supercharger to save fuel, then reconnect it later (to ram someone off the road!).
I used to think: is that real? Would probably work with an electrostatic clutch but how well would fuel/ air mixture get through the blower when it wasnt spinning? Bearing in mind it looked like a standard 6/71 blower or whatever?
|
|
|
|
|
I think engine size is a result of fuel tax. More tax = small highly stressed engines or diesels, less tax the opposite. This applies in most countries I have visited or lived in.
|
your probably right Old Navy,in the future we will probably see more mondeos and vectra"s etc with boarded up side windows to save on a new windows tax
|
a new windows tax
>>
Dont give them ideas, they are probably watching, (GCHQ?)
|
My understanding from past justification of the turbo route was also to be able offer higher output engines (or power a larger car with the same base engine as a mid range car) was that it saved the cost of developing a large capacity engine which may only be produced in small numbers. That used to be the rationale I heard behind SAAB and its reliance on the same Triumph derived 2 litre engine - they could not afford to develop another engine for such modest production numbers.
I'd be interested from Aprilla and others in the industry as to how expensive it is to design a new engine? Is it that remarkable from any other engine ? ie do you have to design from scratch or is it easy to take an off the shelf design and extrapolate/change where necessary?
|
|
|
|
The Americans had a phrase for saying the same thing: "There's no replacement for displacement"
But they did go overboard at times. I was recently given a small book, 'The Observer's Book of Automobiles', full of data for cars on sale in 1976/77. I wasn't aware AMCs were on sale in the UK, but the Pacer according to the book had a max output of 90bhp from its 3.8 litre inline six. That's dire.
The head shaking scene to Bohemian Rhapsody in Wayne's World inside one of these is probably trying to make it go quicker!
|
"max output of 90bhp from its 3.8 litre inline six"
But plenty of torque, no doubt, in keeping with its intended audience, in the land of 55mph limits.
IIRC, the current Corvette has a fairly softly tuned 5-litre lump that allows 36mph/1000 rpm - much preferable, IMHO, (and probably more economical) than a forced induction engine half the size.
|
|
|
|
The Americans had a phrase for saying the same thing: "There's no replacement for displacement"
roughly translated into English that phase means "no matter how hard we try, we cannot extract power from a small engine and have given you this big, thirsty, slow one instead"
I drove across the US in a 1984 Pontiac Parisenne with the 5.0 in 1998. That was quite a cool car, in a barge like sort of way and good at cruising along at about 60mph. But it was slow. I had a MKII 1.8 Golf Driver sat at home at the time which would have blown the Pontiac into the weeds in every measurable aspect of performance and economy, 0-60, mid range acceleration, top speed, braking, cornering, the lot. On a previous trip Stateside I had a Chevy S10 with the 2.8. Now, you would think a 2.8 V6 would be quite lively. WRONG, my Skoda Favorit that I sold to part finance the trip was like a F1 car compared to that and used half the fuel.
Give me a good engine every time over a big one.
Edited by Dynamic Dave on 29/10/2007 at 12:35
|
>roughly translated into English that phase means "no matter how hard we try, we cannot extract
>power from a small engine and have given you this big, thirsty, slow one instead"
Utter carp.
The yanks are perfectly capable of producing highly tuned engines - a guy I knew in Austin was getting 200bhp/litre out of NA Olds 4L V8s - they just don't see the point for driving up and down I-35. Even european and japanese manufacturers have learnt that, given the choice between a 4cyl turbo buzzbox and a normally aspirated V6 or V8 for the same cost, the average american driver will go for the bigger engine every time. For example, even though the US market is critical to Jaguar's survival, the new XF will only be available in the US with the V8. They don't even see significant demand for a V6.
>would have blown the Pontiac into the weeds in every measurable aspect of performance and
>economy, 0-60, mid range acceleration, top speed, braking, cornering, the lot.
Do you really think that comparing a fullsize american sedan with a MkII Golf is a valid comparison? Why didn't you drive across the US in a MkII Golf?
>On a previous trip Stateside I had a Chevy S10 with the 2.8.
The S10 is a pickup for glubs sake.
>Give me a good engine every time over a big one.
Like the Small Block Chevy for instance?
Kevin...
|
|
|
|
|
