Article: adding a turbo to improve efficiency

[bkrell]

Pardon me for being the noob wandering into a 5 page thread, but as an owner of two turbo'd SAABs, I've had a little experience with turbos. From my experience, Johnny's right. Turbos are going to help you make more power more efficiently from a smaller motor but for day to day economical driving, you may not see a difference.

SAAB's been turbocharging production cars since 1978 and they've always played it as an economical way to make good power from a small engine. They have a rather simple boost/vacuum gauge in the instrument cluster that shows a three color coded range. The needle points to green (the economical range) when there is a vacuum. Slight boost puts the needle in the yellow range and red for higher boost. It's always been my experience that as soon as the car is taking advantage of boost pressure, economy starts dropping.

And no, turbo power isn't free. There is excess back-pressure when compared to naturally aspirated cars and that backpressure is, of course, produced from the exhaust having to be routed through the turbo vanes. That backpressure creates ridiculous heat that has to be dealt and can lead to other issues.

SAAB and VW have both dealt with sludge issues b/c of the mix of heat and extra blow-by generated by the higher compression inside the cylinders combined with low crankcase oil volume. When SAAB redesigned their bread and butter 4 cyl turbo engine in 1998 they used a simpler ring design in an attempt to provide less friction. This redesign used a PCV system completely inadequate for the job and all that turbo-induced blow-by deteriorated the oil and PCV hose rubber pretty quick, clogging oil pickup screens and leading to oil starvation. SAAB had to issue a total of 6 pcv upgrade kits before they halfway got it right for owners of 99-2003 model cars.

SAAB also tunes the ECUs to run ridiculously-rich when on-boost to avoid detonation. For certain, it's nice to have a 2.3l engine that makes 211 lb-ft of torque and 185 hp (and 260 hp in current trim on the 9-5) but when cruising my turbo is off boost and the manifold is in vacuum. And my best mpg figures are from using pretty much zero boost.

Just my personal experience. I'm no master mechanic by any means.

[sickpuppy318]

Hey all, this site is frickin awsome!
I learned some stuff about efficiency maps while learning to use thermal efficiency maps of specific turbos to match them with your engine. I was trying to pick a turbo for my tercel.

The deal with the graph in post 60 is that anybody can look at a graph and interpret what it is supposed to mean. But what can you do with it?

That graph was ment to demonstrate the efficiency enhancing characteristics of a turbocharger. The two different conture maps correspond to equal displacement engines. The Y axis is not torque, but rather a theoretical estimation of cylinder pressures.

Someone said that the efficiency island is for unachievable cylinder pressures at cruising speed. That the extra efficiency cant be realized in real driving, and that the only efficiency gains from a turbo would be from downsizing the engin.

That is only partly true... A large enough engine would see no gain from a turbo, because the pressures with enhanced efficiency would produce too much torque to cruise with (BMEP = 4pieTorque/VOLUME).

But if the engine had such little volume that in order to achieve cruising level torque output the engine needed extream cylinder pressures (above efficiency island), then the addition of a turbo would increase curising efficiency. Flat ground, lower RPMs.

Or so says the graph in post 60.

Here's my interpretation of the graph (in color!), except here i've graphed two instances of Pressure at cruisng torque at each engine speed. Think infinitly slow accel., infinitly close to steady state torque. Torque stays constant (same aero drag or whatever), but i've included two different engine volumes. Now the graph showes four engines, NA/Turbo X Big on bottom and small on top. The voluminous engine has low pressures at its steady torque level. The tiny engine has high pressures, so high that they're inefficient. The turbo would increase efficiency for the tiny one.

Say your metro had tiny hockey puck wheels that enabled it to cruise slowly but in its best BSFC. Or fifth gear dissapeared. And to boot you had a big billboard on the roof. In this instance the turbo would increase efficency without downsizing the motor.

Or maybe you were just trying to eco-mod your old 22R powered motorhome. Here a turbo would increase cruise efficiency and maintain part throttle efficiency, without downsizing the engine.

[mikeBURRITO]

ok. here's the scoop, I am getting a new car because right now I have a 1996 plymouth neon SOHC. I am starting to think about keeping it and making it a long drive car, but it's only getting 30 mpg hwy. I've read that the engine itself can handle about 6-8 psi for everyday driving, so in order to not chance the catastrophic engine failure, im looking for a small turbo that will max out at maybe 4-5 psi. will this put me at about 40mpg? because that's what im thinking. and im aware of the inherent cost of a turbo, but im hoping to save that in gas. any suggestions?

[California98Civic]

Originally Posted by mikeBURRITO

ok. here's the scoop, I am getting a new car because right now I have a 1996 plymouth neon SOHC. I am starting to think about keeping it and making it a long drive car, but it's only getting 30 mpg hwy. I've read that the engine itself can handle about 6-8 psi for everyday driving, so in order to not chance the catastrophic engine failure, im looking for a small turbo that will max out at maybe 4-5 psi. will this put me at about 40mpg? because that's what im thinking. and im aware of the inherent cost of a turbo, but im hoping to save that in gas. any suggestions?

Subscribed. Interesting idea. Unfortunately, as I recall, the article that began this thread does not give guidance about the output of the turbos tested.

[brucepick]

From my memory of the first posts in this article - from some of our best educated people - adding a turbo by itself does not guarantee increased FE. To get the benefit, you will need either a smaller engine or taller gearing, and probably both.

Assuming the same engine, you'll get a wider power band, starting at much lower rpms. You'll also get more fuel into the engine at any given rpm/throttle combination. In fact, that's the purpose of a turbo - to get more fuel into the same size engine, thus more power out. European auto makers used turbos to get around displacement limitations in some countries - most notably in Italy. Italy had a 2 liter maximum for many years, larger engines had a huge jump in taxes/fees. So they sold 1.999 liter turbo'd engines in Italy, made by several Euro manufacturers (Volvo for one, with the "Polar" 240).

Going back to the earliest posts, I think that to get a FE benefit from a turbo you will need either a smaller engine or taller gearing, and probably both. When you consider the cost of the turbo + installation effort, I dunno that it's worth it. You might do much better with just maybe one size bigger tire (taller, not wider), a lighter foot, and a good FE gauge like an MPGuino.

Then there's the issue raised by bkrell in post #101 above. One of several really - but with the turbo in play, you're pressing the whole system to the max possible, stressing the oil, creating more blowby, etc etc.

If you want to find an engine that's about 1/2 the size of the original and turbo it, you might get improved FE. If you found one 2/3 to 3/4 size you could just install it with decent gearing without a turbo, and have a car that gets good FE.

Just my .02

[jakobnev]

om my memory of the first posts in this article - from some of our best educated people - adding a turbo by itself does not guarantee increased FE. To get the benefit, you will need either a smaller engine or taller gearing, and probably both.

There was also the option of having the intake valves close later. Which is just within reach of DIY. How this helps isn't completely obvious, but to make a long story short: It increases the mean cylinder pressure on the intake stroke relative to the same during the compression stroke, hence lowering pumping losses.

[Ladogaboy]

@ mikeBURRITO: The others here have nailed it. The fuel economy to be had from a turbo is obfuscated by a number of factors, but ultimately, that fuel economy is nothing more than a byproduct of being able to get more power out of a smaller engine.

Take my car, for instance. It is able to produce more power out of 2.0L of displacement than many naturally aspirated engines two to three times its size. The real benefit for me comes when the turbo itself is not active. Essentially, because the engine will only be at 30-40% load (turbo not active) when I am cruising, I'm getting the same mileage as a naturally aspirated 2.0L engine.

Unfortunately, when under >50-60% load, I'm getting the same mileage as those larger displacement motor. As the turbo spools, more air (and consequently, more fuel) is being rammed into each cylinder. This produces gobs of power, but also eats up much more fuel. Also, as bkrell had pointed out, manufacturers tend to run a rich fuel mixture to prevent detonation/knock. My car tends to run a 10-11:1 air fuel ratio (AFR) under boost... far from stoich (14.7:1 AFR, or 1 Lambda), and very far from lean burn efficiency (I've been told it is ~ 15.2:1 AFR or 1.2 Lambda).

On a side note, this is one of the tracks I will be going down in terms of maximizing my highway efficiency. Under freeway cruising speeds (goal is to maintain 30-40% load at those speeds), I'm going to attempt to increase my timing to 42-44* and AFR to ~15-16:1. This can be done because my turbo starts to spool at ~3,250 rpm, and my car's gearing allows my to cruise at 65 mph at ~ 3,000 rpm. Again, I'm only using the 2.0L motor, not the additional, virtual displacement that the turbo provides.

In your case, if your ultimate goal is freeway efficiency/economy, I'm not sure that converting your current motor to a turbocharged motor is the right way to go. You'd be surprised at how much one of those conversions costs to do properly, and even if done properly, your potential mileage gains are dubious at best. In my personal opinion, turbocharger kits should be reserved for those cars whose engines were designed for turbo chargers. I've seen too many blown engines and lackluster results from naturally aspirated engines with turbo kits thrown on.

[econsatty]

as far as gearing goes, would a turbocharged engine with different gearbox that favors lower rpm shifts provide better fuel economy? the reason I ask is the Saturn SOHC transmissions all have taller, wider spaced gear ratios than the transmissions mated to the DOHC engines (which had a closer set of gears to make it feel sportier). I own a SOHC saturn with the wide-spaced, taller set of gear ratios. If I were to swap the SOHC engine (which was matched with the taller gearbox) with a DOHC engine (designed for the shorter, closer gear set), the DOHC engine would probably have poor fuel economy compared to the SOHC engine because I would constantly have to stab the throttle to accelerate. The point I'm trying to get to is this: would a turbocharged DOHC accelerate well with the longer gears and wider ratios? The engine wouldn't need to rev as high to make power to accelerate, and could shift sooner. The longer gears would keep the RPM lower while accelerating and (maybe?) allow for better fuel economy. Does anyone have any thoughts on this? Can I upgrade the engine with a better-breathing DOHC and still get the SOHC fuel economy? I would HATE to say goodbye to the 46MPG I get with my pokey SOHC.