The truth about small capacity turbo engines

If you grew up before the Internet, a 1.5-litre engine does seem insufficient to the task of dragging the sorry arse of a mid-sized SUV all over town. Perception ain’t reality, though...

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TRANSCRIPT

This episode is rated ‘P’ for physics, ‘M’ for mathematics, and ‘R’, for reality. Sorry about that.

Still, this should be almost as fun as a room full of monkeys sucking filthy German exhaust gas.

If you want to know about the merits or otherwise of this plague of new-fangled small turbo petrol engines, you’ll just have to put up with that.

This report is completely self-serving - because I’m entirely sick of writing people bespoke ‘War and Peace’ responses to this kind of question:

 
"The new Mitsubishi Eclipse Cross has a 1.5 engine with a turbo and seems to do the job but we are a little worried about that it may be underpowered although he fuel economy seems good.” - Newton P.
 

You can cross out ‘Eclipse Cross’ and insert similar widespread consumer reservations about Honda’s 1.5 turbo engine and Hyundai-Kia’s 1.6T. For a non-technically literate person, they do seem pretty small engines. I get that.

I guess if you grew up before the Internet, a 1.5 or 1.6-litre engine does seem insufficient to the task of dragging the sorry arse of a mid-sized SUV all over town. Perception ain’t reality, though.

While you were perhaps unconscious, Rip van Winkling away the past three decades, in addition to that pesky Internet, internal combustion underwent something of a revolution, with the introduction of electronic fuel injection, variable valve timing, direct injection, and turbocharging.

Just to smash those rose-coloured glasses of retrospectivity, a 1990 Honda Civic had a 1.5-litre engine and a carburettor (remember those?). It developed 73 peak kilowatts and 122 peak Newton-metres.

If we fast-forward almost three decades, the capacity hasn’t changed but the output certainly has: The 1.5-litre turbo engine in the Honda CR-V is 140kW and 240 Newton-metres.

The performance has doubled in 30 years as a result of engineering braniackery - another non-word that cries out to be included in the lexicon, stat. Today’s 1.5 turbo engine goes about the same as a three-litre atmo engine, back in the day before Dirty Harry needed a Zimmer frame to grease punks with that enormous six shooter.

FASTER FAILURE: THE TRUTH ABOUT WEAR & TEAR

 Accelerated life testing in R&D land is a better predictor of longevity than you looking at the spec sheet and applying 'logic' from 1980...

Accelerated life testing in R&D land is a better predictor of longevity than you looking at the spec sheet and applying 'logic' from 1980...

People also ask me about wear and tear. Such a small engine, wound up so tight. Surely it will wear out quicker. This seems logical but is also bullshit. These things are integrated designs.

Manufacturers do a great deal of accelerated-life testing, and they put engines through hell in R&D, to ensure all the bits work together to deliver reasonable durability. They don’t always get it right, but mostly they do. So there’s no reason to infer premature failure from a 1.5 or 1.6-litre turbo engine.

I’d bear in mind, however, that turbos are driven by exhaust gas, so they get very hot.

Exhaust outlet temperatures near the manifolds can be in the region of 850 degrees C when you’re up it for the rent. The driven side of the turbo is swimming in high-temperature gas, and it’s lubricated by engine oil.

HOW TO KILL A TURBOCHARGED ENGINE (BEST TWO WAYS)

So it’s fair to say turbos are hard on engine oil. And for this reason, I would not be disrespecting the service interval. Blowing up a turbo is far more expensive than just getting the car serviced on time.

Nor would I be shutting the engine down immediately after a hard run. If you do that, you consign the lubricating oil in a hot turbo housing at the time the car is shut down, to (literally) a living hell.

There’s a section of road I use routinely for road-test evaluations. It’s a 200-metre vertical climb over about 4000 metres of twisty road near my joint, with about a dozen places where you can use full throttle out of the bends. And there’s a cafe right at the top. Called ‘Pie in the Sky’, appropriately enough.

FLOW VERSUS HEAT

The wrong way to do this is to thrash the car to the top of the hill, then pull in, shut down and order a double espresso. It’s a really good idea to idle the engine for a couple of minutes to bleed that heat out of the turbo housing, no matter how caffeine deficient you are in the moment.

 
I was chatting forced induction with the girlfriend (she’s a mechanical engineer – no need to look at me like that) and I found this statement online: 'Though turbos do increase back pressure on the engine, they derive power primarily from otherwise wasted exhaust heat as opposed to exhaust flow pressure.' Surely it is exhaust gas flow that does the work? Can you edumacate us with one of your awesome video efforts?” - Kelvin
 
 Hyundai 1.6T engine in Tucson (pictured) as well as Kona and i30 SR out-performs the (also excellent) Mazda SKYACTIVE 2.5-litre atmo engine

Hyundai 1.6T engine in Tucson (pictured) as well as Kona and i30 SR out-performs the (also excellent) Mazda SKYACTIVE 2.5-litre atmo engine

Happy to. Having an engineering girlfriend is very brave. Still, chicks who can solve differential equations are generally very hot where it counts. They’re usually would up pretty tight. (When you hit the ‘release’ valve it’s often spectacular. That’s what I heard...)

So let’s see if Kelvin’s engineering hottie approves of this: That quoted statement is bullshit. Or at least disingenuous and myopic.

Heat makes engines work. Fuel burns. Heat is produced. It is the effect of that heat on the pressure of the gasses present at the time of combustion (and afterwards) that makes all engine functions occur.

However, at a deeply fundamental level, heat doesn’t make turbos work. The driven side of a turbo is a fan. Fans are driven by flow. You can put a turbo in the oven; it’s not going to spin. Flow is caused by pressure differential.

Here’s how this really works: Turbos increase the volumetric efficiency of engines, by jamming more air into them. More air means you can burn more fuel. More fuel means you can derive more work at the crankshaft. Energy in the exhaust flow drives the turbo.

And that’s why these 1.5 and 1.6 turbo engines perform about the same as a good 2.5-litre atmo engine like the Mazda 2.5 SKYACTIV engine in the Mazda3, Mazda6 and CX-5. The outputs: very similar.

You can look at those engines and even tell how much boost is happening (ballpark) 2.5 divided by 1.5 is about 66 per cent increase - so there’s about 0.6 or 0.7 bar worth of peak boost out of the turbos. A bar is about one atmosphere.

So the boost is about 10 psi if you think in Imperial. (Or about 500 millimetres of mercury if you want to be a totally obtuse engineering nerd or girlfriend.)

So that’s not all that much boost, which means you can use a pretty small turbo, with low rotational inertia, which spools up pretty fast and mitigates lag, and helps with durability generally.

WHAT TURBOS REALLY DO

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Because I am a totally obtuse nerd, and I sympathise with other totally obtuse nerds (one of society’s most repressed groups) and because I sense, deep in the Force, that you want to get in touch with your own totally obtuse inner nerd, here’s the detail of how that turbocharger does its thing:

When the spark plug fires, the mixture burns. Pressure increases in the cylinder and the piston gets shoved down the bore. Because: engines.

The exhaust valve opens before the piston reaches bottom dead centre, and there is still a lot of pressure in the cylinder (relative to atmosphere). So if you’re an engine designing propellerhead, you could hypothetically use that pressure to push the piston even more.

But if you do that by opening the exhaust valve later, the engine would need to do more work (as a pump) to eject the spent exhaust gas. And engines are not pumps. (But they do incur pumping losses.) So you have to choose a ‘Goldilocks’ point in time to open the exhaust valve, which balances work derived against pumping losses.

And the upshot is that the exhaust rushing past the valve is still expanding quite rapidly as it vents down the pipe to atmosphere. Pressure on the engine side of the turbo is much higher than the end of the exhaust pipe. Therefore: you get flow.

For exactly the same reason, water emerges from the garden hose when you open the cock.

OVERLY ENERGETIC

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The exhaust gas has mass, and flow with mass equals all the energy you need to drive a turbo. Actually, there’s too much energy in the flow, at times, which is why turbo installations have wastegates - which is just a fancy-schmancy name for a bypass valve.

Wastegates are a great idea because they stop the turbo working so hard that it pumps up the pressure to the point that something melts and/or breaks - because, that’s bad.

So I guess you could say that heat drives the flow. Fundamentally, heat makes engines work. Combustion drives the temperature up from maybe 50 degrees C on the inlet side to as much as 850 on the outlet side. That’s about a fourfold increase in absolute temperature - resulting in a fourfold increase in pressure.

Plus the boost. Plus the compression ratio. (These are really rough estimates - so, ballpark only.) There are also about 50 per cent more molecules of gas in the exhaust products than on the inlet side - because, chemistry - so that drives the flow, too.

But you have to bear in mind that the biggest gas component on both the inlet and outlet sides of an engine is just atmospheric nitrogen along for the ride - and warmed up substantially.

I’m wondering: Is your brain hurting yet? If it is: Welcome to my world.

CONCLUSION

So in short, on turbos, flow makes them go. Obviously the turbo does create resistance to exhaust flow (back pressure) but it’s clearly worth it on the input side (otherwise they wouldn’t bother). The proof is: Turbo engines go better than atmo engines of the same volumetric capacity.

They key takeouts for those of you still with me, hoping to let your inner engineering nerd off the chain: An atmo engine: Suck, squeeze, bang and blow. A turbo engine: Pump, squeeze, bang and blow. Yes!!! I don’t know which I enjoy more. Both are quite uplifting if done diligently. I sincerely hope engineering hottie concurs.

A one-point-something turbo engine is sufficient to drag the sorry arse of a mid-size SUV all over town. There’s no doubt about that. Get it serviced on time, and idle for a couple of minutes after driving hard, before shutting down - if you know what’s good for you.

Finally, don’t disrespect chicks who can count without using their fingers - you might be pleasantly surprised.

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