Time Waster
Veteran
Nah! Steam is real as they say!
(Or was that steel is real? )
(Or was that steel is real? )
Engine Displacement.
- Thread starter Time Waster
- Start date
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figbat
Slippery scientist
- Location
- South Oxfordshire, UK
Turbo lag is largely gone through the various means mentioned above - some cars also use the hybrid e-motor to fill in for dips in turbo performance. Ultimately though a nat-asp engine is more responsive and more linear. My favourite engine so far was the 3-litre inline-6 fitted to the BMW 130i I had - a colleague described the i6 configuration as "God's own engine layout".
Profpointy
Legendary Member
Are you sure about that? Surely if the turbo isn't giving much boost then you'd in effect be running a low compression ratio engine, which is a bad thing in terms of efficiency of heat engines. There may be other factors, but still ...
figbat
Slippery scientist
- Location
- South Oxfordshire, UK
Agreed - the turbo is there to do a job and when it isn't doing that job it's getting in the way.
Jameshow
Veteran
But you wouldn't drive around at 1000rpm more like 1500 when the turbo's kicked in.
Nothing like a saab 900 turbo or Volvo T5 fir turbo lag!!
rogerzilla
Legendary Member
The reduced displacement turbo engines got good mpg in the older tests but not so much in real driving. A 10:1 or 11:1 compression ratio and a turbo don't work unless the engine runs rich when on boost. The NEDC test never included any hard acceleration at all, so the turbo engines never came on boost. The WLTP test is supposed to be more realistic.
Profpointy
Legendary Member
I think it's worse than it merely being an overhead because the engine proper is optimised for being blown
fritz katzenjammer
Der Ubergrosserbudgie
- Location
- Deepest Darkest Canada
I’m reading this from my comfy chair here in North America… there’s a 263hp, 3.5 litre V6 in the drive that’s considered a relatively small engine. Ford doesn’t sell cars over here, just trucks, except the Mustang. The biggest selling vehicle in the US of A is the F-150 pick up.
what a screwed up place.
what a screwed up place.
I have a 2011 F150 Raptor. I don't drive it a lot, but when I do, it does put a smile on my face.
Drago
Legendary Member
- Location
- Suburban Poshshire
Indeed. We're living in houses and they're discussing the latest trend in caves.
fritz katzenjammer
Der Ubergrosserbudgie
- Location
- Deepest Darkest Canada
I’ve got motorbikes for that need… there really is nothing quite so nice as too much power.
I learned to drive behind the wheel of a Buick Wildcat with a 390hp V8… it really was stupid.
Gillstay
Über Member
I can remember driving a 205 1.9 Gti and then comparing it to a mates car in the states. To get around in something as quick as the 205, they were doing 10 miles to the gallon. Just nuts.
Incase it's not all been written already, one of the main causes of inefficiency in traditional petrol engines is pumping losses.
Because the air:fuel ratio has to be within a narrow range to burn correctly, the engine's power output is controlled by limting (throttling) the airflow into the engine (and metering fuel accordingly though injector duration).
Hence, under any conditions other than wide open throttle the pistons are drawing against a vacuum, which consumes power. Engines are most thermally efficient at wide-open throttle.This is one of the reasons why, all things being equal smaller engines are intrinsically better on fuel for a given power output - so the same 30kW required to push a car along at a steady 60mph will use less fuel if it's coming from a 500cc engine with the throttle wide open than a 2000cc engine with the throttle mostly closed.
There are also other related factors at play that also harm efficiency at part throttle; such as the reduced thermal efficiency associated with the the lower dynamic compression ratios that come with low volumetric efficiency / partial cylinder filling, and poor combustion quality due to insufficent mixture atomisation due to low inlet charge velocity.
Of course the major drawback to going smaller on displacement is less absolute power output, however supercharging (be that with a mechanical supercharger or turbo) can be used to increase output. So now you can potentially have the same output from a smaller, more efficient engine.
The downside is that higher cylinder pressures are required, meaning greater demands on the engine's components and secondary systems such as its cooling and lubrication systems, as well as the greater complexity from the supercharging system. Running high boost pressures with a turbo can also bring issues with throttle response and various other drivability concerns.
Funnily enough when I was at uni I considered this to be the obvious, desirable route before manufactures had really adopted it as a strategy. Now, decades later it's clear to see the practical drawbacks such as reduced engine life.. and from an ownership perspective I'd certainly favour a larger naturally aspirated engine for its relative simplicity, longevity and reliability.
Because the air:fuel ratio has to be within a narrow range to burn correctly, the engine's power output is controlled by limting (throttling) the airflow into the engine (and metering fuel accordingly though injector duration).
Hence, under any conditions other than wide open throttle the pistons are drawing against a vacuum, which consumes power. Engines are most thermally efficient at wide-open throttle.This is one of the reasons why, all things being equal smaller engines are intrinsically better on fuel for a given power output - so the same 30kW required to push a car along at a steady 60mph will use less fuel if it's coming from a 500cc engine with the throttle wide open than a 2000cc engine with the throttle mostly closed.
There are also other related factors at play that also harm efficiency at part throttle; such as the reduced thermal efficiency associated with the the lower dynamic compression ratios that come with low volumetric efficiency / partial cylinder filling, and poor combustion quality due to insufficent mixture atomisation due to low inlet charge velocity.
Of course the major drawback to going smaller on displacement is less absolute power output, however supercharging (be that with a mechanical supercharger or turbo) can be used to increase output. So now you can potentially have the same output from a smaller, more efficient engine.
The downside is that higher cylinder pressures are required, meaning greater demands on the engine's components and secondary systems such as its cooling and lubrication systems, as well as the greater complexity from the supercharging system. Running high boost pressures with a turbo can also bring issues with throttle response and various other drivability concerns.
Funnily enough when I was at uni I considered this to be the obvious, desirable route before manufactures had really adopted it as a strategy. Now, decades later it's clear to see the practical drawbacks such as reduced engine life.. and from an ownership perspective I'd certainly favour a larger naturally aspirated engine for its relative simplicity, longevity and reliability.
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Would that have been an N52 - the last of the naturally aspirated 6s..? I very much fancy a Z4 with one of these in.. not sure if you're aware but it controls air intake to the engine by controlling valve lift and duration rather than using a throttle; which seems to bring economy benefits. How did you find yours on fuel?
As a configuration the straight 6 is arguably unexciting but ticks a lot of technical boxes such as its intrinsically balanced nature.
