Hypermiling a turbo GDI engine (VW TSI)
 

I've been hypermiling for almost 15 years now (I recall hanging out at CleanMPG around 2007-2009 or so), but I just decided to create an account here because my current car has been leaving me puzzled. I posted something similar at the CleanMPG forums back when I bought the car, but apparently the forum is not very active currently.

As everyone knows, the latest trend is in downsized, turbo direct injection engines. I bit the bullet last year and bought a VW up! TSI sporting a 1.0 l, 3-cylinder turbo engine with 105 hp and a manual transmission.

While I'm getting nice figures (about 12-13 km/l tank averages with E100 fuel, all-city driving, with very short, 5-10 km trips), I get the feeling I could be doing better. Especially from the kinds of mileages that I see reported elsewhere

I obviously follow the usual hypermiling tips: reduce speed, properly inflate tires, remove weight from the car, time traffic lights, avoid putting yourself in a position where you'll need to brake, no air conditioning, etc.

As for driving style, I use P&G with engine on (trust me, this car doesn't like to do EOC -- I'm quite familiar with EOC as I used it on my previous cars), try to stay in a 1700-2700 RPM band more or less, and target 70-80% of max load during pulses, which means using some boost. I installed an analog boost gauge on the car, which connects to the MAP sensor of the car (not to OBD-II), and seeing as the car can reach about 0.7-0.8 bar of boost, I try to run it at 0.2-0.4 bar of boost. I've seen people say you should avoid boost like the plague on turbos, but on the other hand, the BSFC maps that I've seen appear to contradict that advice -- sadly I've yet to find one for my engine.

A few days ago I decided to look at the throttle position reading using an OBD-II app for my smartphone and found that, at about 0.2 bar boost or a little bit over that, I already reach the WOT condition at 88%. Further pressing the throttle pedal will take boost to 0.7-0.8 bar like I said, while the throttle position stays fixed at 88% -- I believe the wastegate valve may be controlling the exact amount of boost. I've always understood that a partially closed throttle reduces efficiency through pumping losses, so it appears to me the 0.2 bar boost for pulses may be a good choice.

Also, I've got a lambda gauge (rich/lean) on my OBD-II app, and monitoring it as well as PID 03 appears to indicate that my car stays in closed-loop, lambda=1 mode even at WOT and high boost values of 0.7 or 0.8 bar. I've mostly tested this at low to mid RPMs, which is the range of interest to hypermilers anyway. From this it appears that fully flooring the car (at least at low RPMs) shouldn't harm fuel efficiency.

I welcome comments on what I'm currently doing and advice on what I should be doing better. Also, if anyone who advocates avoiding boost on turbo engines could explain to me the technical reasons behind this, I'd also be grateful, because I'm really not convinced that I should be avoiding boost -- at least that's not I see in BSFC maps for turbo engines.

Did you ever notice any influence of weather to the fuel-efficiency?


You might remember when direct injection was quite a rocket-science feature. Then, either resorting to a large-capacity intercooler (or water injection) or to enrich the air/fuel ratio was a requirement to prevent knock. Remember when Hyundai introduced the 1.0 turbo to the previous-generation HB20 retaining the port-injection and was plagued with excessive fuel consumption? No wonder it has turned to direct injection for the current generation, the same way it had been with any other overseas Kia or Hyundai with the 1.0 turbo engine before its introduction to Brazil.

A few years ago, I was driving a rental car in Finland. A Golf with the 1.2L TSI. When doing P&G, I noticed the instantaneous fuel consumption was the same at WOT and other high-load pedal positions. So I just floored it for the pulses! I can't say if it was the most efficient way, but it was fun and easy. This was all at lower RPM.

Definitely, although as I mentioned, I mostly do short trips, so a big factor is the starting temperature and how long the engine takes to heat (although the engine in my car is designed for quick heating, but still, since trips are short, it will spend a non-negligible amount of time in a cold condition).


I'm not sure I get the point you're trying to make here. Could you be more explicit?

Maybe you're saying turbo was an issue in the past? Like I said, however, instrumentation indicates that my car runs stoichiometric regardless of what I do, so unless the OBD-II data is not accurate, then enrichment is not an issue -- indeed, I've thought of trying to read the voltage of the oxygen sensor directly somehow, but haven't gone to the trouble of doing that yet.

Do note that I see changes in lambda sometimes: for instance, after a pulse, when I shift the car into neutral, the mixture is enriched for a few seconds, then overshoots a little into lean territory for a couple of seconds, and then settles into stoichiometric again. So it's note a case of a "broken" ECU reporting a constant lambda=1 all the time.

But perhaps there are reasons to avoid boost other than fuel enrichment? That's what I'm looking to be enlightened with.

With direct injection, it's mostly pointless trying to get out of boost.

I can't speak for your specific vehicle, but engines under boost have higher cylinder pressures. To prevent detonation (also called "knock"), the typical strategies involve going rich and retarding ignition timing.

Going rich reduces combustion temperatures by dumping in extra fuel which won't be burnt to make power, but instead just absorbs heat and is flushed out the exhaust.

Retarding ignition timing starts combustion later, and effectively dumps part of the useful energy that could have been extracted from combustion, out the exhaust.

If your engine has the proper fuel and spark maps to take advantage of E100's higher knock resistance, it may not need to go rich or pull any timing under boost, but that isn't a given.

Turbo engines usually have lower static compression ratios, which makes it so they don't need to dump as much fuel or pull as much timing under boost, but it also makes them less efficient outside of boost.

~

12-13km/L all-city driving doesn't seem bad at all to me, frankly, but there aren't many (any?) cars sold in the US which can match those figures without hypermiling, that aren't hybrids.

Nice observation, which reminds me: are these higher cylinder pressures harmful to the engine? In that case, if I'm trying to balance fuel savings with maintenance cost, trying to make the engine last as long as possible, maybe I should prefer less over more boost? Unless more boost is especially efficient.

I live in Brazil and the absolute majority of cars sold here are flex fuel straight from the factory (and to be clear, my car is one of those). Indeed, you'll find both E100 and E27 on nearly every gas station in the country.

You do remind me, however, that I should perform the same tests on E27 as well. Maybe I'll switch fuels for the next tank to test that.

The results are as shown, though: lambda=1 and closed loop operation under all conditions that I tested (save while in DFCO, evidently, and when switching the car into idle after a pulse).

My car's engine in particular has a 10.5:1 compression ratio. Not sure you'd consider that low; my previous car was NA and had the same ratio.

Still, is it the case that the thermodynamic efficiency increases with boost? I've always thought that, in a way, boost achieves the same effect as increasing the compression ratio. If, at a given moment inside the engine, there is a full 1 bar of boost over atmospheric pressure, then isn't that equivalent, in terms of efficiency, to having a NA engine at WOT with twice the compression ratio?

In that case, and assuming the car is indeed running stoichiometric under all conditions, then I would assume the most efficient operating point would be at max boost.

Indeed these are good figures (the local equivalent to EPA figures are 9.6 km/l city and 11.1 km/l highway), and I can get even better figures when conditions help.

For instance, today I took a somewhat larger trip (about 10 km each way), most of which was on the peripheral highway around my city. Recall it's summer here, and I don't use A/C. Because it's Sunday and there were few cars on the road, it was safe to P&G around 50-60 km/h, sometimes going a little higher. Even with a small patch of city driving, a few traffic lights, etc. I was able to achieve 19 km/l according to the trip computer, which after correction should be closer to 17 km/l, but still, it's an excellent figure.

The reason I'm looking for ways to improve is that I've seen people report figures as high as 25-28 km/l (of E27) on the highway, and these people probably aren't taking any heroic measures to save fuel (i.e. they're probably targeting a speed of 100 km/h or more, A/C on, etc.) Now I'm well aware that there are differences on the quality and performance of two engines coming out of the same plant, due to e.g. manufacturing tolerances (and never mind that I haven't yet put 10.000 km on my car, so the engine is probably a little rough still). Also, knowing my fellow countrymen, I wouldn't be surprised if they're "rounding up" the numbers a bit; probably using trip computer values rather than actual measurements at the pump; claiming a record best, once in a lifetime figure is an average; etc. Still, maybe there's some truth to these figures, in which case I have a lot to learn.

Probably not. The turbo has a lifespan, and being in boost more frequently might increase piston ring wear somewhat, so maybe it'll start burning oil a little sooner. I don't have a good reference to go by for small turbo VW engine lifespan though.


Even under boost? That's surprising, but suggests much lower economy losses under boost.




Same as my car, approximately. The Insight stock engine was 10.8:1. That was 20 year old port injected tech though.

Mazda's SkyActiv direct injected engines are 13-14:1, but they're on the upper end of what's typical.

OTOH, higher compression does show diminishing returns. I think at ~10:1 it's something like 2.5% efficiency for an additional point of compression, but less going from 12:1 to 13:1.


I would think so, yes. There are some other confounding factors though.

For example with my K24 engine (10.5:1 compression), I have to pull timing at WOT below 3000rpm, or I get knock, even with 93 octane fuel. Undoubtedly timing is being pulled with cylinder pressures literally twice as high in a turbo engine. Some of those efficiency gains are being lost due to not being able to use MBT ignition timing.

Turbo boost is also not *entirely* free, it's just far less costly than a supercharger. The exhaust gases leaving the cylinder have to push through a turbine, and that steals a bit of energy from the piston trying to evacuate the gases from the cylinder.


With my Insight in perfect functioning order, with some small aero mods, before my engine swap, I was able to achieve around 100mpg @ 50mph, in good weather, which is around 43km/L. Even with the much bigger and less efficient ~240hp 2.4L K series engine under the hood, I'm still seeing ~26km/L at lower highway speeds.

I'm certain there's some more you can squeeze out of your car. I'll be looking forward to seeing what you come up with!

Yes, even totally flooring the car. At least with E100. And like I said before, under some specific situations I can see it report lambda values other than 1 (under DFCO and coming out of a pulse), so it's not a case of a broken ECU that only reports lambda=1.

I'm reading lambda from PID 34 (hex)/52 (dec), which is reported as supported by my car. But if there are suggestions of other PIDs which are more reliable for reading lambda, I'm all ears.

It looks like ignition timing is something I'll have to learn about. I'll start monitoring that gauge during my drives and see if I can learn something.

Wow, I'm thoroughly impressed. Does it use a manual transmission? Are these figures for E0, or E10, or what?

My previous car was a Honda Civic Si (not sure if the engine was a K20Z3 or K20Z5), and I believe I got, only once, 20 km/l out of it, driving very very slowly, using E27 -- that car wasn't flex fuel. In the city, 12 km/l was an excellent mileage.

Just for the record, there's some help from your car's aerodynamics: according to Wikipedia, the 1st generation Insight had a Cx of 0.25, and the frontal area (if it's as simple as multiplying width and height) is about 2.30 m^2, so a Cx*A = 0.574. According to one source that I found, the up! has a Cx of 0.367 and a frontal area of 2.08 m^2, so Cx*A = 0.763 (about 33% more than your Insight). Also, I'm avoiding very high tire pressures on my car: I'm using about 35 psi -- not sure about you.

Regardless of these differences, these are very impressive figures.

Have you documented your driving style somewhere in the forum? If not, could you very briefly summarize it here? I know these are very different cars and engines, but I could probably learn a lot.

So I went out for another drive today, paying attention to ignition advance -- PID 0x0E (hex)/14 (dec).

So indeed, at lower boost, ignition advance is higher (more degrees before TDC), while at higher boost, ignition advance is lower (fewer degrees before TDC). Either way, while pulsing, it's always positive (before TDC). I assume none of this is news.

So you claim ignition is retarded (pushed closer to TDC) to avoid knock, and that makes sense: if you ignite too early, there is the risk of the flame front meeting the piston head while it's still compressing rather than expanding.

What I'm trying to understand is how that "dumps part of the useful energy that could have been extracted from combustion, out the exhaust". I imagine early ignition, when the flame front meets a compressing piston, besides being destructive (through knocking), would also waste useful energy by pushing the piston head in the opposite direction of its movement, therefore doing negative work. But if the flame front meets the piston head when it's already expanding, then isn't that doing positive work as expected? Where exactly is useful energy being dumped out the exhaust?

Sorry if it's a stupid question, but I really want to understand this. It would help me understand why my car could have a BSFC sweet spot at part load, and why it would be better to pulse at lower boost pressures rather than higher ones.