My thoughts on low fuel consumption mobility.
Discussion
I've recently been doing a bit of long-distance travelling in my VVC-converted S1 Elise and, with the mindless attention to speeding detail of the Victorian Police (Australia), it involves long stretches of driving at a constant 100km/h. To compensate, the Elise returns rather reasonable fuel consumption figures, but the mind does wander and begins to wonder what it would take to do something really spectacular in the mpg stakes.
Now, over a tank full, my Elise is returning just sub-6L/100km (5.95-5.97) which at 100km/h is 3000rpm. At around this point, the VVC K-series is good for about 295g/kWh*, so my calcs yield a roadload power at the flywheel of around 14.6kW
(*I just happen to remember the test cell data from calibrating the production K-series engines in years gone past...)
So, now for some maths and a few thought experiments.
If I were to re-gear the engine to achieve the best possible BSFC point, it would be cruising down at 1500rpm, 90kPa manifold pressure and the BSFC would be nearer 245g/kWh. Actually the VVC K-series best point is less than 240, but only when producing too much power. Realistically, cruising at 1500rpm would also be infeasible, but it's an interesting datapoint.
This re-ratio'd configuration would yield a fuel consumption of less than 5L/100km, or pretty much heading into diesel territory.
OK, so if I could substitute the engine for a really, really good small diesel or perhaps an atkinson-cycle gasoline engine of some form, what would it do then? Say 200g/kWh was the new, and I think still realistic, BSFC target. This eeks things out to nearly 4L/100km - getting exciting, but not really very close to Dr Piech territory yet!
So, we've probably exhausted the limits of modern engine technology at this specific fuel consumption level, we'll now have to look at reducing roadload parasitics, both aero and mechanical. Rob Collingridge lists the S1 Elise's Cd and CdA on his website as 0.408 and 0.653 respectively. This means we can calculate out the aero component at 100km/h as 8.6kW, leaving the remainding 6kW as tyre and powertrain losses.
If we gave the Elise the full slippery treatment and got the Cd down to, say, 0.28, that alone would reduce aero power by nearly 2.7kW and improve the basic fuel consumption to 4.9L/100km, or just under 4.0L/100km when geared optimally. With an 200g/kWh eco engine upgrade, it would reach down to 3.3L/100km, assuming the new engine was sized to deliver this ever decreasing amount of power - no small challenge in itself, since engine fuel consumption doesn't scale all that well due to surface/volume ratio effects.
We could of course, start improving rolling resistance (the tyres on my car were already the narrow spec 185's and 205's in Michelins running at 0.5Bar over spec) and driveline/braking friction. There's a good 6kW to find there, but I don't have any meaningful grasp on tyre theory and the rest would be just guesswork on my part, so we'll leave that for now. But suffice to say, an Elise with new clothes and a revised engine could be a pretty impressive fuel miser.
So, as a final data point, I feel we must return to the good Doctor's XL1, which is indeed a fine benchmark to consider. Googling gives it's roadload power as 6.2kW at 100km/h, if press blurb can be relied upon. This is presumably aero and mechanical friction altogether, which seems realistic. Powered by a standard VVC K-series, or equivalent, that would give us under 2.6L/100km, or 2.1L/100km operating at it's best point. Replaced with the "lowest fuel consumption 6.2kW engine known to man", this would be just over 1.7L/100km.
At this point we're nearing the laws of diminishing returns and lousy driving technique would easily obliterate any or all of these improvements together.
But, it does demonstrate that we should be able to engineer a car that easily does 2L/100km in the cruise with known modern technology and, possibly even a relatively simple, low-cost engine. I favour a relatively large displacement, undersquare, OHV V-twin running an atkinson cycle, where weight and friction can be easily optimised and parts count can keep it cheap. By adopting a HEV powertrain, maximum IC engine power output could be (speed) limited to 20-25kW or so covering most of the parasitic energy losses, with an electric motor-generator to deliver the required acceleration, peak gradability demands and also offer some useful low-speed EV-only operation.
By keeping it's EV-only range short-ish, the overall mass and cost of the vehicle could be kept down to make it affordable and fun to drive, even on low-friction skinny tyres.
So, that's my recipe for a future of personal mobility. Land's End to John o'Groats on less than half a tank (25L) of 95RON? I think it's doable, but does it meet PH requirements?
Now, over a tank full, my Elise is returning just sub-6L/100km (5.95-5.97) which at 100km/h is 3000rpm. At around this point, the VVC K-series is good for about 295g/kWh*, so my calcs yield a roadload power at the flywheel of around 14.6kW
(*I just happen to remember the test cell data from calibrating the production K-series engines in years gone past...)
So, now for some maths and a few thought experiments.
If I were to re-gear the engine to achieve the best possible BSFC point, it would be cruising down at 1500rpm, 90kPa manifold pressure and the BSFC would be nearer 245g/kWh. Actually the VVC K-series best point is less than 240, but only when producing too much power. Realistically, cruising at 1500rpm would also be infeasible, but it's an interesting datapoint.
This re-ratio'd configuration would yield a fuel consumption of less than 5L/100km, or pretty much heading into diesel territory.
OK, so if I could substitute the engine for a really, really good small diesel or perhaps an atkinson-cycle gasoline engine of some form, what would it do then? Say 200g/kWh was the new, and I think still realistic, BSFC target. This eeks things out to nearly 4L/100km - getting exciting, but not really very close to Dr Piech territory yet!
So, we've probably exhausted the limits of modern engine technology at this specific fuel consumption level, we'll now have to look at reducing roadload parasitics, both aero and mechanical. Rob Collingridge lists the S1 Elise's Cd and CdA on his website as 0.408 and 0.653 respectively. This means we can calculate out the aero component at 100km/h as 8.6kW, leaving the remainding 6kW as tyre and powertrain losses.
If we gave the Elise the full slippery treatment and got the Cd down to, say, 0.28, that alone would reduce aero power by nearly 2.7kW and improve the basic fuel consumption to 4.9L/100km, or just under 4.0L/100km when geared optimally. With an 200g/kWh eco engine upgrade, it would reach down to 3.3L/100km, assuming the new engine was sized to deliver this ever decreasing amount of power - no small challenge in itself, since engine fuel consumption doesn't scale all that well due to surface/volume ratio effects.
We could of course, start improving rolling resistance (the tyres on my car were already the narrow spec 185's and 205's in Michelins running at 0.5Bar over spec) and driveline/braking friction. There's a good 6kW to find there, but I don't have any meaningful grasp on tyre theory and the rest would be just guesswork on my part, so we'll leave that for now. But suffice to say, an Elise with new clothes and a revised engine could be a pretty impressive fuel miser.
So, as a final data point, I feel we must return to the good Doctor's XL1, which is indeed a fine benchmark to consider. Googling gives it's roadload power as 6.2kW at 100km/h, if press blurb can be relied upon. This is presumably aero and mechanical friction altogether, which seems realistic. Powered by a standard VVC K-series, or equivalent, that would give us under 2.6L/100km, or 2.1L/100km operating at it's best point. Replaced with the "lowest fuel consumption 6.2kW engine known to man", this would be just over 1.7L/100km.
At this point we're nearing the laws of diminishing returns and lousy driving technique would easily obliterate any or all of these improvements together.
But, it does demonstrate that we should be able to engineer a car that easily does 2L/100km in the cruise with known modern technology and, possibly even a relatively simple, low-cost engine. I favour a relatively large displacement, undersquare, OHV V-twin running an atkinson cycle, where weight and friction can be easily optimised and parts count can keep it cheap. By adopting a HEV powertrain, maximum IC engine power output could be (speed) limited to 20-25kW or so covering most of the parasitic energy losses, with an electric motor-generator to deliver the required acceleration, peak gradability demands and also offer some useful low-speed EV-only operation.
By keeping it's EV-only range short-ish, the overall mass and cost of the vehicle could be kept down to make it affordable and fun to drive, even on low-friction skinny tyres.
So, that's my recipe for a future of personal mobility. Land's End to John o'Groats on less than half a tank (25L) of 95RON? I think it's doable, but does it meet PH requirements?
Whilst i agree with your assumptions, data and general working, my experience suggests your approach for a "supper frugal" ICE powered vehicle are somewhat undermined in the real world by the average person being a terrible driver!
The single biggest advantage with EV's is that their regenerative capability means they can be driven poorly or in more stop/start (less cruise) conditions and still return significant fuel economy. A second advantage is because this energy flow between the cars mass (KE) & it's fuel storage system (battery) is bi-directional, it offsets a large proportion of these vehicles higher mass (by around 65% as that is the typical round trip efficiency.
So whilst an ICE can deliver excellent economy in a carefully driven cruising scanario, in the real world it's a lot less clear cut for the average driver! (Skilled drivers can, and do, get ridiculous ranges out of modern cars when driven with ultimate economy in mind: 1626_miles_to_a_tank_Passat)
However, the killer advantage for pure EV's is not anything to do with the cars themselves in my opinion! Because, what an EV does is to make the cars energy consumption and pollution during it's useage SEP (Someone Else's Problem). For an OEM that is a massive massive advantage, and one that will see EVs continue to be pushed and developed by those OEMs, at the cost to other technologies.
OEMS marketing department already, erroneously, trumpet things like "ZERO pollution" for their EV's, when of course what they actually mean is "Zero Tailpipe pollution". They still pollute, it's just the nasty stuff comes out of a big chimney in Yorkshire, rather than at the back of the car......
What is interesting is that one thing Lotus could do, and do well, is to manufacture a "Eco" Elise for 2014. They a have the manufacturing and platform flexibility to quite cheaply develop say a low drag Elise, with maybe the 1.0 Ecoboost engine from Ford power products. (With a boost tweek, that's ~150bhp and 200Nm, significantly more than an original Elise)
The single biggest advantage with EV's is that their regenerative capability means they can be driven poorly or in more stop/start (less cruise) conditions and still return significant fuel economy. A second advantage is because this energy flow between the cars mass (KE) & it's fuel storage system (battery) is bi-directional, it offsets a large proportion of these vehicles higher mass (by around 65% as that is the typical round trip efficiency.
So whilst an ICE can deliver excellent economy in a carefully driven cruising scanario, in the real world it's a lot less clear cut for the average driver! (Skilled drivers can, and do, get ridiculous ranges out of modern cars when driven with ultimate economy in mind: 1626_miles_to_a_tank_Passat)
However, the killer advantage for pure EV's is not anything to do with the cars themselves in my opinion! Because, what an EV does is to make the cars energy consumption and pollution during it's useage SEP (Someone Else's Problem). For an OEM that is a massive massive advantage, and one that will see EVs continue to be pushed and developed by those OEMs, at the cost to other technologies.
OEMS marketing department already, erroneously, trumpet things like "ZERO pollution" for their EV's, when of course what they actually mean is "Zero Tailpipe pollution". They still pollute, it's just the nasty stuff comes out of a big chimney in Yorkshire, rather than at the back of the car......
What is interesting is that one thing Lotus could do, and do well, is to manufacture a "Eco" Elise for 2014. They a have the manufacturing and platform flexibility to quite cheaply develop say a low drag Elise, with maybe the 1.0 Ecoboost engine from Ford power products. (With a boost tweek, that's ~150bhp and 200Nm, significantly more than an original Elise)
Yes, I agree with your point about driving style being a big determinant for ultimate fuel consumption capability. This is where I think a hybrid powertrain could shine. The range of an ICE and the control of an EV. With a hybrid powertrain, all the combustion engine needs to do (largely) is compensate for roadload losses - the driver doesn't actually need direct control of it.
The problem is that right now neither EV's nor high MPG (diesel) ICE's are cheap to make.
I would propose a couple of features to create a series/parallel hybrid powertrain that could offer a relatively cheap overall vehicle - low parts count engine, electronically regulated dog gearbox, no clutch/EV launch, limited battery capacity requirements. Screw all that together into an XL1-style vehicle and I reckon you'd have something relatively inexpensive and massively capable small vehicle at a price that people would be prepared to buy.
The problem is that right now neither EV's nor high MPG (diesel) ICE's are cheap to make.
I would propose a couple of features to create a series/parallel hybrid powertrain that could offer a relatively cheap overall vehicle - low parts count engine, electronically regulated dog gearbox, no clutch/EV launch, limited battery capacity requirements. Screw all that together into an XL1-style vehicle and I reckon you'd have something relatively inexpensive and massively capable small vehicle at a price that people would be prepared to buy.
Edited by AER on Monday 3rd November 10:51
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