Technical guides Diesel & Petrol

[dieselV6]

2..5TDI 6-spd manual gearbox wins hands down in constant 140mph/225km/h autobahn run, the 2.0CR TDI 170bhp cannot really do it.

As I said earlier, for anything under ~124mph / 200km/h, 2.0CR will run fine and win on fuel economy, no question about that. But at higher speeds, 2.5TDI is far less stressed. There are no miracles here, smaller engine has to pump more air and pumping costs energy. Turbochargers are far less efficient at extracting exhaust energy than we are led to believe, Main fuel economy benefit from turbocharger is that you can run a smaller engine most of time, and then supply enough power when needed (assuming you do not need it often). But in a constant high power demand scenario such as high speed driving for 3 hours, a larger engine works out better. The old adage "there's no replacement for displacement" still holds here. That's why for a moment I seriously considered a petrol V6 Mk2 Superb as a replacement.

Here are my numbers. Autobahn part of my car's mileage is about 40k miles, so I'd say these are repeatable.

You mentioned 17l/100km on a 140CR at full power? That is a lot. My 2.5TDI uses 12l/100km average, this is from 80+ trips, and that's at 138mph/222km/h cruise. Because of slowing down on roadworks, local speed limits etc, average trip speed works out to about 125mph. With 69l (vented) fuel tank, and leaving 8l reserve, this is enough to cover about 510km/315 miles at about 124mph/200km/h average and in about 2.5 hours. I still would prefer 80l fuel tank as it would allow me to always cross Germany without refuelling. Diesel is moe expensive in Germany than in neighbouring countries, plus Mk1 Superb is comfortable enough for ~3hrs-~3.5hrs between stops.

Above figures do not include wind, which can be a major contributor to burning fuel at high speeds. I remember 1 trip where due to strong and constant headwind I had to travel 10mph+ slower than usual, and still burned an extra tank of fuel (that's 69l) over a 900 mile trip.

So based on the numbers, my 2.5TDI now looks actually more efficient at high speeds than I thought. This, together with peak speed of about 148mph makes the Mk2 Superb look inadequate as a replacement, hence I started looking at 2.0CR 170bhp equipped Octavia 3 and Rapid. Though it is now officially announced that Mk3 Octy does not have any spare wheel at all (tyre inflation kit), and the Rapid has no 2.0CR engines as of yet. I don't think the local Skoda dealership will see me for anything other than spare parts at least for the next 3-4 years, if ever. Nor any VW/Audi dealership for that matter, as it is current VAG corporate policy to get rid of spare wheels and shrink fuel tanks.

Edited 17 January, 2013 by dieselV6

[Kiwibacon]

You need to look at a BSFC chart some-time. The whole "less-stress = lower fuel consumption" is *******s.

Diesels are most efficient at full load. Smaller engines running more boost use less fuel. Which is why cars, trucks, tractors, locos, in fact all internal combustion engines are being downsized and boosted.

Your 2.5 V6 has ~50% more internal friction than a 4 cylinder, also more heat loss from the cylinders which doesn't move the car forwards. It cannot overcome these efficiency penalties at any point in it's operation.

My 17 litres/100km is the worst the (calibrated) instantaneous readout can show in 6th gear. It is not an average, none of your figures are obtained in the same way.

But overall, where do you get the idea that the 2.5 v6 is an untouchable powerhouse from? Even if it was 190hp stock (I can only find figures for the 150hp version), a 170cr will be producing not far off that stock and can be mapped to produced a lot more with no mechanical changes.

The 2.5v6 is not a well loved engine. Between fuel consumption, camshaft wear, fuel pump failure and lack of power it is not the one to have.

[Kiwibacon]

Last I checked, spare tyres were one of the easiest accessories to install. Whether a car comes with them or not doesn't matter. All that matters is they can fit one.

ALL skodas sold in my country have a spare wheel.

[dieselV6]

Diesels are most efficient at full load and low to mid range rpm and at full load Above 3800rpm or so, diesel's efficiency goes down. It probably would not if the exhaust was sized for efficiency at max rpm not mid range. My numbers are at least real, and I can tell you that from comparing 1.9 and 2.5 at 125mph (my first trips after buying, the Mk1 Superb, I used Mk1 Octavia earlier), 2.5 used less fuel because 1.9 was working full throttle full time and 2.5 was working at half throttle.

2.5TDI we are discussing here was 163bhp stock, is 190bhp now. 2.5TDI is not an"untouchable powerhouse", I can name easily an engine better thant that, it is 3.0V6 TDI in pretty much every guise since its introduction by Audi, not to mention the excellent 3.0 diesel straight 6 from BMW, in my opinion the best diesel for comfort, economy and power output combined.

Put simply, 2.0 is too small engine for the job. Just like in my wife's Roomster the 1.6CR is too small engine even for UK driving, compared to the 2.0CR (that unfortunately is not sold with a Roomster)

Spare is only easy to put in the car if the boot floor has full size spare wheel well. Yes current Skodas have full size well, but new ones (Yeti and now Octy 3) have lowered boot floor to inflate boot capacity figure. So the best you can fit there, if anything, is the thin space saver, and that is unsatisfactory.

I'd be happy with no spare if the boot was 640mm deep and there was a threaded hole in the side of the boot to mount spare wheel into. Until that happens, putting spare on boot floor and taking 2/3rd boot floor space with it is a lame option as it makes arranging any other stuff to carry difficult.

[erikiri]

Take a look at the picture gallery there, for me it looks there is normal spare wheel 205/55R16 below the boot floor: http://octavia3.cz/recenze/

[dieselV6]

That looks promising although the picture is not clear enough to see if it is full size or rather one of those "expandable" spares that Audi and Porsche use nowadays.Notice the spare wheel well is much wider than the wheel itself at opening, that's why I mention it.

For UK, the specification states tyre inflation kit or optional space saver spare (thin one) . So I guess even if it is a full spare in this picture, I'd have to import the car from Ireland to get it, again.

Many thanks for the link, if you see a more detailed picture of the spare wheel bay anywhere please post. If it is possible to fit full sized spare in there, that just leaves the thimble fuel tank issue on the 4x4 and vRS versions I'd be looking at anyway.

Edited 18 January, 2013 by dieselV6

[xman]

Diesels are most efficient at full load.

Diesels are most efficient at full load and low to mid range rpm and at full load

Both wrong I'm afraid....with electronically controlled injector tech the manufacturer can place the engine max efficiency point almost anywhere. Its normal for truck engines to have a max efficiency point at part load/engine speed related to 56mph for instance.

Note Max engine efficiency does not equate to min fuel consumption as transmission losses/rolling resistance/aerodynamic drag etc have to be taken into account.

You need to look at specific fuel consumption and some graphs

[dieselV6]

Keep looking at graphs, and I will keep looking at my wallet and fuel put in the tank on like for like trips. As I said, at 125mph a 1.9TDI, although still burning less than a petrol engine, was burning more than a 2.5TDI, same wheels, same gearing, 300kg lighter car, and keep in mind 2.5TDI is less efficient engine than 1.9 not just on account of its size, but also compression ratio and injection pressure. What you posted are engine efficiency maps, not car efficiency maps. A car has air intake and exhaust that is sized by beancounters and emission regulations, not sized for optimum efficiency.

Cars are built for average usage and the exhaust is simply not sized for optimum continuous operation at high rpms. This dramatically increases pumping losses in the system. Have a look at turbocharger effciency vs exhaust backpressure graphs, it makes a very interesting reading.

Unlike your graphs, my fuel economy numbers tend to be averaged over hours of autobahn run at a time. Probably 25k-28k night autobahn driving in Octavia, and another 38k-42k in the Superb. See my car's mileages in the sig. Octy and Superb's mileages are 60%+ across Europe.

I can understand why you think otherwise when just looking at graphs, but that is exactly what is broken with current fuel economy testing, instead of real life, components are tested in an artificial scenario.

Edited 18 January, 2013 by dieselV6

[xman]

You obviously dont read what I write..

Note Max engine efficiency does not equate to min fuel consumption as transmission losses/rolling resistance/aerodynamic drag etc have to be taken into account.

Your statements made previously were about engine efficiency not "car efficiency" - whatever that means.

You need to buy a small light, low profile, low Cda car with a large fuel tank - a diesel Ferrari with optional wing tanks perhaps? Dont forget to specify the skinny high profile eco tyres - you'd be surprised at what a difference they make....

Me, I'm sticking to cheap and cheerful - for me, depreciation is the biggest cost by far. :beer:

[dieselV6]

A diesel Ferrari? Which one? There are so many to choose from

Seriously, a diesel vRS or 4x4 Octy Mk3 with underbody protection would do just fine, if only fuel tank was larger. Aux tank would be an option were it not for insurance, I looked into it once (for Octy Mk1).

Whatever I said on effciency, I quoted high speed motorway numbers of car with one engine vs the other, it's real engines put in real cars we are talking here, 2.0 is OK till ~120mph, above that it is better to use something bigger, 2.5-3l.

Last time I worked depreciation on the Octy RIP, it was 30% lower than fuel cost alone (not to mention tyres/pads/filters etc).

On Superb, due to high fuel prices, depreciation will be 1/3rd or less of total cost of ownership if it makes it to 10yr mark.

If you buy brand new factory order cars, it pays to run them until they die...

Edited 18 January, 2013 by dieselV6

[k.young]

Take a look at the picture gallery there, for me it looks there is normal spare wheel 205/55R16 below the boot floor: http://octavia3.cz/recenze/

That definitely looks like a full size spare.

[dieselV6]

I'll believe it when I see the size numbers on the tyre

[k.young]

Oh ye of little(very little) faith!!! :bandit:

[dieselV6]

I'd have a lot more faith if Skoda UK simply listed "full size steel spare wheel with jack and tools", as Skoda Ireland inevitably does for most cars

[Kiwibacon]

Xman and DieselV6.

Here is a diesel engine BSFC chart.

The zones on it show how many grams of diesel are required to deliver each kw of crank power. Lower numbers show higher efficiency.

As you can clearly see, best efficiency is found at or near full load, as you can also see, once you are past max torque, the efficiency drops with rpm.

This is nothing do with exhaust size or electronically controlled injectors. This is simply a function of the burn speed of diesel compared to the piston travel speeds. As the pistons travel faster, the diesel burning cannot build the same peak pressure and efficiency drops. Engines with longer stroke develop peak torque and peak efficiency at lower rpm, engines with shorter stroke (1.6tdi) develop peak torque and peak efficiency at slightly higher rpm.

The reason higher loads give better efficiency is also simple. As load and output gets higher, the internal friction becomes a smaller percentage.

Hopefully this will help you both understand a little of diesel engine operation.

As you can see, the idea that a bigger engine provides better fuel economy is completely false.

As you will also come to realise in time, electronic injection control cannot shift best efficiency points anywhere in the load/speed range. It is dictated by the same engineering principles that it always has. Electronics cannot change that.

Truck engines have peak efficiency at cruising speeds because the vehicle is geared to run that way. Again it has nothing to do with electronic injection.

I (and I'm sure all other readers) fail to see how comparisons between a 1.9tdi and 2.5v6 tdi are in any way evidence that a 2.0tdi with more power than the V6 is unsuitable.

[dieselV6]

Why won't you drive a 2.0CR car at full throttle (at 4k rpm or thereabouts) then run a 3.0CR car at same speed as 2.0CR did, each for at least an hour instead of looking at charts? These graphs you are posting are for lab tested engines, with little or no restriction at exhaust. Then compare actual real car fuel consumption on apples to apples basis, just like I compared apples to apples with 1.9TDI injection pump engine to 2.5TDI injection pump engine at around 125mph cruise / 110mph average.

Furthermore, please go to Garret turbocharger pages and seek out turbocharger efficiency maps, the key design parameter for any turbocharger is pressure ratio. Any exhaust restriction immediately affects pressure ratio. I tested this for myself when I replaced pilot catalyst with a decat pipe on the Superb, turbocharger spins up about 200rpm-300rpm earlier and goes to full pressure noticeably faster. Just because of removing some restriction from exhaust (and there's still main catalyst, resonator and muffler there).

Once again, that is exactly what is wrong with modern fuel economy testing - people looking at single system component graphs rather than actually driving the complete cars.

Or in extreme case, do you think the map you posted would look the same if you attached a reducer and a 10mm diameter exhaust pipe to the engine under test?

Addendum: even on the MEP graph you posted, an engine working at 4k rpm and full power would use ~223g/KWh, while a 30% bigger engine working at 4k rpm and 75% power (thus delivering the same power) would use about ~218g/KWh. That's in lab conditions, exactly as you like it , without taking DPFs, resonators and mufflers on the exhaust into account.

2.5/1.9 = 1.3 ...

Edited 18 January, 2013 by dieselV6

[Kiwibacon]

I am quite sure I understand more about turbocharging and exhaust restriction than you do. There are dozens of vehicles worldwide running turbochargers I have selected, most of them Garrett. I even build my own turbocharger combinations to suit my engines and driving style.

From that I can heartily say that none of your concerns there are valid. I guarantee the 2.0 CR is a more efficient engine that burns less fuel at every operating point than your 2.5v6.

1.9 to 2.5v6 is not apples to apples. Your 10mm exhaust idea is silly.

Modern fuel economy testing is testing of complete cars. Please stop annoying us with your misunderstandings.

[dieselV6]

Oh, I see you ran out of facts, so it is now personal remarks time. Not to worry, would you please care to comment on data taken from your own map that you yourself posted?

>>>Addendum: even on the MEP graph you posted, an engine working at 4k rpm and full power would use ~223g/KWh, while a 30% bigger engine working at 4k rpm and 75% power (thus delivering the same power) would use about ~218g/KWh. That's in lab conditions, exactly as you like it , without taking DPFs, resonators and mufflers on the exhaust into account.

2.5/1.9 = 1.3 ...<<<

And you are still missing the point. 2.0CR is not powerful enough to provide cruising speed of 138mph that my current Superb does, no matter how efficient. Given a choice of modern engines in the VW range, I would take the 3.0CR V6, or at the very least the 184bhp 2.0CR as in the upcoming Octy 3 vRS (and in a lighter/more aerodynamic chassis that can do with 20bhp less to reach 135mph based on currently issued figures for Octy 3 and Superb). In fact, now that it looks likely that Octy 3 will have a full sized spare, it looks to be my preferred car as soon as the vRS comes out. If 4x4 has the 184bhp unit, it will be even better, and I'll address the small fuel tank if I have to.

Edited 18 January, 2013 by dieselV6

[Kiwibacon]

Pretty simple.

Your hypothetical bigger engine has a completely different map. Trying to pick numbers off a 2.0cr map to predict 3.0tdv6 performance simply does not work.

I've written it many times, the 170hp CR can be remapped to provide far more power than your 2.5v6 with no hardware changes. The 184hp 2.0CR will out the box provide the same power as your tweaked 2.5v6, remap it and you'll get much more.

Even the 110kw 2.0 has a claimed max speed of 218km/h (135 mph).

[dieselV6]

Would you care to present a 3.0CR map then?

110kW has 135mph only on Octavia, on the Mk2 Superb that we discussed earlier, even 125kW has only 137mph. That's why I started looking at Octy 3 in the first place. 135kW engine in the Mk3 Octy vRS will probably be good for ~144mph or so and then it is enough (except the small fuel tank).

Edited 18 January, 2013 by dieselV6

[Kiwibacon]

I do not have a 3.0tdi v6 map. If you kindly ask VAG they may supply one.

[dieselV6]

That definitely looks like a full size spare.

I have just re-checked updated ETKA:

P1G1 temporary space saving wheel

P1G2 spare wheel (steel) with full tyre

P1G3 spare wheel (steel) with full tyre

Hooray! Even if it means a couple trips to Dublin to get the car I want

[dieselV6]

I do not have a 3.0tdi v6 map. If you kindly ask VAG they may supply one.

Haven't found the 3.0CR map yet, but here is statement on the new bi-turbo 313 bhp V6

http://ae-plus.com/features/-audi-30-v6-tdi-biturbo/page:2

"The best brake-specific fuel consumption is 199g/kWh. Consumption and emissions figures on the NEDC have yet to be homologated, but provisionally they go from 6 litres/100km and 158g/km CO2 to 6.4 litres/100km and 169g/km respectively, despite 7% longer gearing. “Our base engine was designed for low CO2 emissions – the Biturbo uses more fuel but has excellent acceleration,” said Bauder. “The boosting system increases weight and thermal inertia. And the double post-injection, to increase exhaust-gas temperature, means a slight deterioration in fuel efficiency. In future we want to compensate for these increases, in all engines.”

So peak is the same, gearing is indeed longer for more powerful engine, both these together would result in 3.0CR using less fuel than 2.0CR at high revs and 2.0CR close to full power. Again, even under lab conditions, at speeds approaching 2.0CR power limit the 3.0CR will be more efficient (and much more refined)

[Kiwibacon]

You can't make that claim with the information available (single data point). You are guessing about the rest of the map and using that guess to support your hypothesis.

The 28 year old 3.9L tdi truck engine I have runs 210g/kwh at peak efficiency. Peak means very little.

[dieselV6]

Equally, you cannot make your point about the 2.0CR being better than 3.0CR at 130mph speeds without presenting both maps rather than just the 2.0CR one.