Fuel consumption: Old v New

[Ferrer]

These days you see ever impressing official fuel consumption figures from seemingly impossible high performance cars. The recently unveiled BMW M5 is a classic case in point, it homologates 9,9l/100km in the combined cycle, which is over 4 litres every 100kms better than its predecessor.

Tightening emission regulations, size increases and new equipment all play against the ultra modern direct fuel injected, down sized, turbocharged engines. Everyone knows that the official cycles have little to no relation at all to everyday driving, so do those improvements actually translate to real life conditions?

To be brutally honest, I have limited experience with modern down sized engines, pretty much the only one I've been able to use thoroughly is the Delta 1.4 T-Jet we have in the family, and even it isn't that ground breaking. Yes, it does have a turbo and reduced engine capacity, but no direct fuel injection and the basic engine design is quite old.

But the fact of the matter is that in (demanding, undeniably) usage the modern Lancia struggles to get below double figures and isn't that far off the (very, by comparison) old Audi 80 2.8E, a car that has twice the engine displacement and is a relatively big executive saloon.

I am aware that I am comparing oranges and apples here, but I'm trying to illustrate a point. Will the modern day Audi A4 2.0T see a significant improvement over my old six? Or are we being fooled. And that's actually only discussing fuel economy because in terms of sound, enjoyment and pleasing driving characteristics there's really no contest.

As far as I've been able to determine, the closer you are to the ideal conditions the more sense they make. But once you are out of their "comfort zone" they use petrol as car of its power always has done, downsizing or not.

And apparently it isn't limited to petrol engines either, because the downsized diesels are also having a hard time keeping with their old relatives, as soon as the going gets tough. And here with notable loss in performance, may I add.

So, are we actually going forwards or backwards?

[henk4]

what is the weight of your 80 compared to a modern A4?

[Ferrer]

I don't know, 1300kg or so? And the new A4 may be around 1500kg perhaps?

It's certainly a bigger (and safer) car, but it would be interesting to compare actual interior dimentions.

[henk4]

I don't know, 1300kg or so? And the new A4 may be around 1500kg perhaps?

It's certainly a bigger (and safer) car, but it would be interesting to compare actual interior dimentions.

it is all the safety an luxury stuff, that make economising cars an uphill battle. I am sure if my C3's engine would be fitted into an AX, 30 km/l would be easily achievable.

[MilesR]

This morning's newspaper made a comparison between an HR Holden (large car) from 1966, and the new Holden Cruze "small" car. The exterior dimensions of the two were all within 5mm of each other. The weight of the HR sedan was quoted as 1,180kg, while the weight of the Cruze sedan is about 1,400kg. Consequently, the classification of one as a small car, and the other as a large car, indicates how much the standards for the classes have drifted.

Naturally, the Cruze has a higher power output, and is faster, despite having a smaller engine (1.8L compared with 2.65L for the smallest HR). The performance is only possible because much more power is reaching the wheels in the Cruze than in the HR. Despite this, the fuel consumption, whether real-world or claimed, is better for the new, faster, heavier car than with the old one.

Engines are becoming more efficient, and, driven gently, a direct injection turbo engine will deliver lower fuel use than a comparable older-generation NA engine. This is partly because direct injection allows a higher compression ratio, which results in a higher percentage of the energy from the fuel being turned into motive power. The smaller displacement also reduces the energy that the engine wastes in keeping itself moving.

However, much more progress could be made if safety standards and market forces were not driving cars to become bigger and heavier. A car as small as a Fiat 500 (the original, not the sacrilegious remake) or the Mini would not be built today due to crash safety standards. In addition, because new cars can go faster, I think they get driven faster. Drive a new car as fast as an old equivalent would have been driven, and the claimed fuel consumption would probably be realistic.

The only direct comparisons that I can make between real and claimed fuel usages are for a 2003 Astra, and my 1993 Daihatsu Mira. The Astra, which claimed 7.6L/100km, routinely achieves less than 6.5L/100km around town, and managed 5.9L/100km on a freeway run, a few years ago. The Mira claimed 4.8L/100km, and routinely managed 4.3L/100km, before a major service was done, and 5L/100km afterwards (not the best service I have ever had).

My conclusions are: 1) yes, progress is being made, but much more could be made if current trends did not result in ballooning weights and sizes, and 2) yes, claimed fuel consumption figures can be matched, or bettered, but it depends upon driving style. I would not expect to match them if I drove a newer car faster just because it is capable of going faster.

[clutch-monkey]

^ great post! do agree.

our 996 GT3 nets the same or better fuel consumption than our much less powerful '78 SC, even with the latter's engine swap helping things.

[csl177]

^^^ +2! Great post and spot on... modern cars are so much more efficient and could be more so, if only the weight added by safety mandates and consumer demanded accessories could be mitigated by lighter materials. The biggest issue with carbon composites in mass auto manufacture are autoclaves capable of high production. Maybe the next best thing will be industrial adhesives that don't require as much tooling.

MilesR, thanks for a refreshingly lucid post. A rare thing from the recent noobsquad. Hope you'll hang around.

[Badsight]

best example of balloning weight is the MP4-12C

despite a radical increase in engineering & manufacturing technology - the super light & tiny v8 engined car is 200 KG heavier than the 3 seater V12 F1

& thats because of more restrictive safety standards that they have to meet 19 years on

[Badsight]

I would not expect to match them if I drove a newer car faster just because it is capable of going faster.

that is basic commen sense

cars have an optimum speed where the fuel consumption is at its lowest

often 80 km/h is more thirsty than 100 km/h in a modern car - they are tuned to give a sweet spot & air resistance squares with speed - so 200 km/h is 4 times the drag than 100 km/h has

air resistance & getting into the power (read air consumption) band of the engine boosts the rate of fuel consumed

gearing & torque curve is what makes 100 cheaper than 80 in most cases where it happens

lugg along in top at too low a revs & the engine is working harder on less torque - less fuel efficient depite their being a lower rate of air going thru the engine & less air resistance on the car

[pimento]

MilesR, thanks for a refreshingly lucid post. A rare thing from the recent noobsquad. Hope you'll hang around.

Note the join date - someone who joined and didn't spam the bejeezus out of the place.. and now gave a very good contribution to the thread.

Badsight, comparing the F1 to the MP12-4-C-11-whatever isn't really apples to apples.. one of the apples had rather more high tech doodadery in its construction and costs three times as much or thereabouts. I'm sure if the price to consumer was equal the weight difference would be much smaller.

[Ferrer]

Great post indeed by MilesR.

Altough I was thinking more in cars that were 10-20 years old, not 40+.

He also raises an interesting point, because cars have gotten quieter and more stable and are capable of much higher speeds comfortably, does that mean we are driving them faster and therefore negating the theoretical fuel consumption advantage?

[pimento]

10-20 years ago the speed limits were the same or higher, but not sure about rates of speeding fines issued or anything like that. Driving may have become more aggressive though which would contribute to negated fuel savings, as well as traffic becoming heavier and such. Certainly according to the car manufacturers we've been heading to a leaner greener future, as better fuel economy has been a headline on most model refreshes in the past while.

[Badsight]

one of the apples had rather more high tech doodadery in its construction and costs three times as much or thereabouts. I'm sure if the price to consumer was equal the weight difference would be much smaller.

not really true

it has a lot to do with economy of scale that allows the MP4 with its even more elaborate construction to be cheaper.

[Ferrer]

10-20 years ago the speed limits were the same or higher, but not sure about rates of speeding fines issued or anything like that. Driving may have become more aggressive though which would contribute to negated fuel savings, as well as traffic becoming heavier and such. Certainly according to the car manufacturers we've been heading to a leaner greener future, as better fuel economy has been a headline on most model refreshes in the past while.

That was the point, the official figures say we are on the right track, but does it actually to translate to real life?

Because I haven't seen it.

I had high hopes for the Delta, but as far as I can tell it uses petrol as a "conventional" 150bhp car does.

[pimento]

not really true

it has a lot to do with economy of scale that allows the MP4 with its even more elaborate construction to be cheaper.

That's a part of it too, but the point still stands. Remember with the F1 that the engine was outsourced to BMW, and given the press that BMW got out of that, I'm sure there some cost savings for McLaren with that situation. That along with the no compromise approach with the gold lined engine bay and whatnot point to a no holds barred approach to the project. The MP4-12C (such a bad name..) doesn't seem to have had quite the same hard core approach.