Fuel consumption of TDI vs. SDI engines at high load

Hello!

In the AMS test for the Golf III (all diesel models) from 1997, I found:
Minimum SDI circulation rate: 3.9 l.
Minimalrunde TDI 66kW, 81kW: 4.0 liters.

In my owner's manual (Golf III, AFN, 81 kW), I read:
Total consumption: SDI 5.1
Total consumption: TDI, 66/81 kW: 5.0.

It seems clear that, at least in the partial load range, there is not much difference between SDI and TDI engines, although...
The SDI might be slightly more fuel-efficient due to the lack of exhaust backpressure.

But how does the SDI perform in the upper load range / full-throttle range?

"When an 81kW engine can easily keep pace with an SDI engine that's running at its maximum speed."
Then I suspect that both engines are burning roughly the same amount of diesel fuel (assuming that...
(they both get enough air).

However, the opinions I found about the search were not consistent.

Quote:

dieselmartin wrote on November 9, 2004: A TDI engine driven in a mode similar to an SDI engine will be more fuel-efficient.

Quote:

kenny wrote on August 24, 2003: But the SDI engine also tends to consume less, at least when comparing a 68 horsepower SDI to a 110 horsepower TDI that I... I have driven both of them for a long time.

Regards, Klaus.

PS: This was inspired by the discussion on efficiency (Ulf et al.), but it exceeded my capabilities.

Hi.

HM the two statements you cited do not contradict each other (in my opinion).

Because: show me the driver of an 81kW vehicle who drives it like an old-fashioned SDI!

I only know one person who drives a G3 variant with 110 horsepower, and he gets 4.2-4.5 liters per 100 kilometers!
I drive a bit more aggressively, and it only has 90 horsepower -> 6 liters [of fuel consumption].
My ex drives her 1.7 SDI (which, it should be noted, is in a Polo, so it's lighter) and gets 5.8 liters per 100 kilometers, and then she's even slower than me.

The subtle, minor issue is "driving in a similar way." I've experienced it myself: if you can, you tend to do it. The 81 kW engine is sometimes used for quick acceleration, and that's when more diesel fuel gets consumed. The SDI cannot accelerate (it only gets faster ), and that's achieved with very little diesel.

Simply because the more powerful one doesn't necessarily mean you lose the ability to save money, I find it to be the better option.

m;

As far as I thought, the efficiency of a turbocharged engine is always higher than that of a comparable naturally aspirated engine. Therefore, the TDI should theoretically have a lower fuel consumption for the same power output.
Especially at partial load, there is still a significant amount of charging pressure (mean pressure, filling, geometric compression ratio - basic compression).
In my opinion, these factors are always higher with TDI engines!
A table showing the consumption/speed would provide more information here.

I once owned an old E30 325i. The consumption diagram was included in the operating manual.

Because: show me the driver of an 81kW vehicle who drives it like an SDI!

I've been driving like that for the past few months (but it hasn't been a traffic obstruction, at most just slowing down slightly before traffic lights ), because I really wanted to get my average fuel consumption below 5 liters, but I couldn't manage less than 4.8 liters, no matter how hard I tried (but I still think that's a very good value for a 1500 kg Passat ). However, when I drive normally, meaning I also enjoy myself from time to time , I need about 5.8 liters, and that extra liter, which is the difference between creeping along and having fun, is something I'm willing to pay for, even with the current fuel prices. But it's still good to know that there are even more ways to save money if necessary.

Hi!

In a paper about diesel engines in agricultural machinery, I once read that the efficiencies of naturally aspirated and turbocharged diesel engines (with the same level of technical development) are, in principle, not fundamentally different, at least not at the optimal operating point.

When it comes to the power-to-weight ratio, things are different: the turbocharged diesel engine scores points due to its extremely lightweight engine and its smaller (due to being turbocharged) engine block.
At very low load levels, the turbocharged diesel engine also performs well, as it essentially operates like an SDI (turbocharger ineffective) and, due to its smaller size, has to overcome less internal friction, meaning a smaller SDI versus a larger SDI, resulting in lower idling fuel consumption.

The last two points naturally influence the overall efficiency of a vehicle, depending on the driving profile (especially when comparing two engines of equal power). However, in my opinion, the advantages of the TDI engine are offset by the higher purchase and maintenance costs. For example, if you compare a 1.7 SDI (AKU) with 60 hp directly to a Lupo 3L TDI (61 hp), the cost difference becomes significant.

Regards,
Holger.

Quote:

However, in my opinion, the advantages of the TDI engine are offset by the higher purchase and maintenance costs. For example, if you compare a 1.7 SDI (AKU) with 60 hp directly to a Lupo 3L TDI (61 hp), the differences become apparent.

Great comparison! You certainly couldn't have found anything more absurd, could you?! The high initial cost of the 3L is definitely not solely due to the engine. Regarding the maintenance (costs) of a regular TDI, I think they're probably not significantly higher than those of an SDI.

The high initial costs of the 3L are certainly not solely due to the machine itself.

...but also in the weight-reduced design of the body. However, this also contributes to the overall efficiency of the vehicle, so the comparison is acceptable in this regard.

At another point, the comparison isn't entirely fair to the 3L TDI: The 1.7 SDI (AKU) belongs to an older engine generation with a distributor pump, while the 3L is a common-rail (PD) engine.

And regarding the maintenance (costs) of a normal TDI, I think it's probably not much higher anymore.

Well, a few more sensors and actuators, plus a heavily stressed mechanical component, and a higher thermal load from the block, pistons, and valves, which requires piston cooling oil, which itself can also fail... that's quite a chain reaction.

Okay, let's say the regular maintenance is the same for TDI and SDI engines. But the failure rates... The probability of failure is undoubtedly higher in TDI engines due to their greater complexity. (Assuming the same level of care and maturity in both designs).

But I don't want to create a divide between these two concepts; both have their merits.

hello
Holger.

...But you do it all the time. What do you want to hear from us? Yes, the SDI is a great car...
Okay, I respect all your concerns about TDI engines. I've actually owned a few vehicles with those engines, including a Polo with an SDI engine. That Polo never had any engine problems. And the TDIs I've owned haven't had any issues either. I also can't say that the TDI was a fuel-efficient marvel. It was quite sluggish and consumed around 5.5 liters per 100 kilometers. "My Golf can do that too, and sometimes my Passat can as well."

Matthias.

Do you mean that the __S__DI isn't a miracle of cost savings?

Otherwise, your sentence makes little sense .

m;

Oh yes, Martin, absolutely my mistake... it should definitely be SDI, sorry.

I'm just going to compare them based on my gut feeling right now.

I have a Passat TDI that's 10 years old, with a 1.9 TDI engine, and a friend has a brand new Seat Arosa with a 1.9 SDI engine.

The SDI has less power and lower fuel consumption, but it's lighter, has a lower top speed, and offers significantly less driving enjoyment. 4.5 liters.

My TDI has more power, more weight, a higher top speed, more driving fun, but also higher fuel consumption. 5.5 liters.

I'd gladly pay a little more for the driving pleasure of a car, even if it's 10 years older and hasn't had any engine problems yet. I'd almost say that my TDI in 'limp home' mode performs as well as an SDI in normal operation.

Everyone has to find what's best for themselves.