Hypermiling a turbo GDI engine (VW TSI)

[Ecky]

Those figures are using E0, so I have a 50% energy density advantage.

I can summarize a few key points I've found in getting good economy with my car:

-Cold starts kill economy. After maybe 15-20 minutes of running, it reaches peak efficiency, but economy can increase by as much as 50% on long trips vs short ones. While there are some things that can help get up to temperature more quickly (grille block, block heater) we often can't choose our driving conditions, so it's important not to get too wrapped up in comparing with other drivers who have different driving conditions

-I find I get better economy the lower I shift. I've heard others report getting better economy by raising their shift points and accelerating briskly, but I personally haven't observed it. I'll throw my car into the highest gear it can take, as soon as I can, if I'm driving for economy.

-On the highway, slow and steady wins. With the stock motor it was very easy to keep it at 50mph and ignore other drivers. The new engine is capable of accelerating much more quickly and requires a lot more self control. I find myself really needing cruise control to keep my speeds down. For my vehicle, above ~50mph / 80kph, economy seems to drop off quickly. Below that, it seems more susceptible to environmental conditions, but slightly higher numbers are possible.

-it probably doesn't need to be said, but around down, conservation of momentum yields large gains.

-Modifications provide very small but incremental improvements that add up. I'm aware of one driver on Insight Central who can consistently get 150mpg+ tanks out of his car. To do this, he runs 120psi+ in his tires (which are mounted on donut wheels), his fuel line wraps around his catalyst to preheat it, and for a while he had completely removed his front brakes (because discs have a lot more drag than drums), plus dozens of other modifications (no mirrors, I think?). I think he's nuts, personally, but it goes to show that there's always more fruit hanging.

That said, even the best mods' gains can disappear behind changes to driving habits. Pick the mods that work best for you, drive as well as you can, and don't sweat the rest.

[Ecky]

Originally Posted by swineone

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.

Great observations!

You're right, it isn't news that ignition timing is more retarded (or less advanced) at higher boost. Just looking at timing numbers though, it's nearly impossible to glean much.

You're right about igniting things too early as having a negative impact on economy. There's a concept, called "MBT" (mean best torque), which refers to the perfect ignition timing at a given load and RPM, to get the most power out of combustion. Ignite the charge any earlier, and you get more negative work, with combustion pushing on the rising piston. Ignite it much later, and the flame is chasing the piston down the cylinder, never actually getting to push on it. It's a continuum, of course, but the idea is that there's a "perfect" time to ignite the charge to get the most power out of combustion.

It sounds like you already know a fair bit of this, so forgive me if I write a lot that isn't news to you, but the reason you can't glean much from just looking at timing numbers is that MBT is different for literally every point on the map, and for every engine. Further, combustion isn't instantaneous - it's a flame front that spreads from the point of ignition, and takes time to complete.

As more fuel burns, the heat (and therefore pressure) in the cylinder rises. If that pressure gets too high before combustion is complete, the remaining unburned fuel can auto-ignite, either in points or all at once. When the two flame fronts crash together, this is audible as a "knock" or "ping", and the resuoting shockwave can damage the engine. It also reduces fuel economy (and power) so even if you build an engine strong enough to handle it, you still wouldn't want it (with some notable exceptions).

Keeping cylinder pressure from getting too high is managed in several ways. Higher "octane" fuel is more resistant to detonation, but typically has some drawbacks (refining, cost, energy density), and what kind of fuel one can get varies from place to place. Others ways include: Reducing intake air temperature, dumping extra fuel to cool the chamber, intercoolers. Some combustion chamber designs are simply better at preventing detonation ("hemi" designs are among the worst, pentroof among the best). Lowering boost (dynamic compression) lowers peak pressure. Reducing static compression. Also, starting combustion later (retarding ignition timing) moves the point at which combustion competes to a later time, when the piston crown is farther down the cylinder, meaning the instantaneous chamber volume is larger when the maximum heat and energy has been released. Unfortunately, retarding timing to prevent knock may also start ignition too late to "capture" as much combustion energy as possible (past MBT), resulting in more combustion energy passing into the exhaust as heat, rather than doing useful work.

Some other interesting bits:

When cylinder pressure is higher (more air and fuel, or boost), the combustion event happens more quickly. One can think of it as the fuel and air molecules as being closer together, so it spreads between them faster and more easily. Likewise with a higher static compression ratio, the flame front literally has a smaller area to fill before complete combustion. Because of this, less ignition advance is needed for MBT (you can start it later because it finishes earlier), and a smaller portion of combustion happens while the piston is still rising, meaning there's less negative work that happens.

This is one of the reasons higher loads are generally more efficient. In most engines, and especially at low load, combustion needs to be started long before TDC. I believe at certain points in my engine's map, I have ignition starting as far as 48 degrees before the piston reaches the top of its compression stroke. At higher loads, it's more like 12-25° depending on RPM (flame speed is mostly constant, so if the piston is moving faster, you have to start combustion sooner).

Different fuels also burn at different rates. Alcohols generally burn faster than gasoline, so they need less ignition advance even with all else being equal.

There are also a few tricks engineers have found with engine design to help with this. One example is an offset crankshaft, where TDC doesn't occur at 0° crank rotation. While it results in an unbalanced engine, it helps sidewall loading on the combustion stroke, but more importantly, it offsets ignition timing so that much less negative work happens. Downsides include a lot more vibration and harshness, and the need for much lower redlines. Possibly engines built this way are also less able to take high boost, though I'm not certain.

There are probably some things I'm forgetting, and some things I haven't explained well, so I might post a bit more later if I think of anything.

[swineone]

Wow. Lots of information from the last couple of posts. It'll take me a while to digest all this. I'll probably come back to them quite a few times. For now:

Originally Posted by Ecky

-I find I get better economy the lower I shift. I've heard others report getting better economy by raising their shift points and accelerating briskly, but I personally haven't observed it. I'll throw my car into the highest gear it can take, as soon as I can, if I'm driving for economy.

My car has a shift indicator, and it appears that, as long as the current speed is such that next gear will be at 1500 RPM, it wants me to shift. For a while now, I've delayed shifting so that I'm closer to 2000 RPM rather than 1500 RPM at the next gear. The turbo lag is especially noticeable under 1500 RPM, with a transition band from around 1500-2000 RPM, and at 2000 RPM I can get full torque.

I was basically afraid of lugging the engine (note this is a 3 cylinder engine), or of being in an inefficient region of the engine map. Maybe I should experiment with following the shift indicator's advice.

Originally Posted by Ecky

-On the highway, slow and steady wins. With the stock motor it was very easy to keep it at 50mph and ignore other drivers. The new engine is capable of accelerating much more quickly and requires a lot more self control. I find myself really needing cruise control to keep my speeds down. For my vehicle, above ~50mph / 80kph, economy seems to drop off quickly. Below that, it seems more susceptible to environmental conditions, but slightly higher numbers are possible.

Do you do P&G, or just cruise at a low load?

I've gotten so used to P&G that I just do it automatically. If I have the car in gear for too long, I start getting jittery. When I got this car I learned how to do a proper double clutch so I can engage back after a glide with minimal jerking. It's just too bad the car doesn't tolerate EOC.

However, I'm not sure how helpful P&G actually is -- and one of the things I had in mind with starting this thread was to figure out the optimal way to perform a pulse. This car is underpowered at low RPMs without boost, and the gearing is very tall (it sure could use a 6th gear), so maybe P&G is not really necessary -- especially considering I can't EOC it.

Originally Posted by Ecky

-it probably doesn't need to be said, but around down, conservation of momentum yields large gains.

This is something I do religiously. Maybe I'm doing everything wrong in terms of how I'm accelerating the car, but I'm sure I make up for it by properly conserving momentum.

[Ecky]

Originally Posted by swineone

My car has a shift indicator, and it appears that, as long as the current speed is such that next gear will be at 1500 RPM, it wants me to shift. For a while now, I've delayed shifting so that I'm closer to 2000 RPM rather than 1500 RPM at the next gear. The turbo lag is especially noticeable under 1500 RPM, with a transition band from around 1500-2000 RPM, and at 2000 RPM I can get full torque.

I was basically afraid of lugging the engine (note this is a 3 cylinder engine), or of being in an inefficient region of the engine map. Maybe I should experiment with following the shift indicator's advice.

The Insight's original engine is a 1.0L 3 cylinder non-turbo with lean burn at part throttle, and very tall gearing - around 1700rpm @ 80kph. Because it lacks a turbo, its efficiency zone may be very different than your vehicle's. Manufacturers very rarely publish BSFC maps, and even doubly so more recently - I don't think I've ever seen one for a modern GDI turbo engine, so it's pretty hard to advise based on published data.

That said, Honda went to great lengths to maximize economy, often at the expense of all else.

Its shift indicator light has some logic which seems to take load into account, but basically it comes on if it could possibly be in a higher gear above idle speeds, or above maybe 2000rpm with accelerating.

This is slightly oversimplified, but when cruising, it leans out to ~24:1 AFR, which allows very high loads when cruising, often above 75%. Even slight grades may need downshifting to maintain speed. It will attempt to hold onto lean burn until maybe 90% load, at which point it drops back to stoichiometric for more power. Below maybe 2500rpm, one of the two intake valves only opens a hair.

The strategy seems to be to get load up as high as possible, as much as possible. It will happily cruise or even accelerate (albeit very slowly) as low as 1000rpm and not suggest a downshift until the pedal reaches the floor, and even then not for a few seconds.

I can't advise if avoiding boost is more or less efficient, but the shift light can probably be trusted.

Originally Posted by swineone

Do you do P&G, or just cruise at a low load?

I've gotten so used to P&G that I just do it automatically. If I have the car in gear for too long, I start getting jittery. When I got this car I learned how to do a proper double clutch so I can engage back after a glide with minimal jerking. It's just too bad the car doesn't tolerate EOC.

However, I'm not sure how helpful P&G actually is -- and one of the things I had in mind with starting this thread was to figure out the optimal way to perform a pulse. This car is underpowered at low RPMs without boost, and the gearing is very tall (it sure could use a 6th gear), so maybe P&G is not really necessary -- especially considering I can't EOC it.

With the stock engine, I found P&G without EOC to provide minimal gains, but P&G is largely a tactic to get load up, and secondly to minimize other engine losses (allow average engine RPM to be lower), and Honda already did a good job getting load up. However, there were significant economy gains to be found if I killed the engine any time there was a downgrade where I could coast without power for more than maybe 5 seconds.

With my new engine I have an even taller top gear, but the amount of torque available makes it such that load isn't very high when cruising, often at or below 25%. I see moderate gains from P&G, and large gains from EOC. I'd say under the right conditions, the improvement can approach 50%. It's kindof a pig, and the less time it spends running, the better.

With my previous car, gearing was very bad for economy. In that one, I could see gains as high as +80%. So, I'd say P&G shows diminishing returns the taller your gearing is, or the smaller your engine is.

Let's hope someone else can speak from personal experience about boost. I wish I could advise on that. With older engines, boost was definitely to be avoided, but that very well may not apply anymore.

[cRiPpLe_rOoStEr]

Originally Posted by swineone

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.

Considering it's built that way at the factory, and the very same engine has been successfully fitted to larger vehicles for a while in other markets, it doesn't seem to be so much of a matter of concern. Besides one engine which was plagued by a defective phasing sensor, the overwhelming majority of 1.0TSI engine failures I'm aware of had been on engines with aftermarket mods.

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.

Did you notice some engines with a smaller unitary displacement at each cylinder have a higher compression ratio than comparable engines with a larger displacement?