Years ago, automakers began phasing out V6 engines and replacing them with small turbocharged four-cylinder units. The promise was the power of a V6 with the fuel economy of a naturally aspirated inline-four mill. But, as we know today, posted fuel economy ratings often don’t match what a driver sees.
And it comes down to how small turbocharged engines operate. Jason Fenske of Engineering Explained produced a handy video to show why said engines normally end up overpromising fuel economy targets.
It begins with how a turbocharger operates. Turbos are spooled up via exhaust gases and shove more air into the engine. More oxygen means more power. But, aside from weight benefits and the simplicity of smaller engines, automakers have to watch out for knock. Knock can occur when a piston compresses the air and fuel mixture in the cylinder. When the spark plug is busy igniting the mixture, the temperature and pressure in a turbo engine can be so great that pockets ignite themselves and colliding flames become a problem that can cause serious engine damage.
That’s where turbo engines become inefficient. To stave off knock, automakers will lower engine compression ratios and a tweaked air-fuel mixture at high loads.
Say the driver floors the throttle. The engine knows it needs to stay away from high-temperature scenarios that can cause knock. So, the engine sends an extremely rich air-fuel mixture to the cylinder. Injecting more fuel actually brings the temperature down to avoid knock. Thus, fuel is used only to keep the cylinder temperature down and create the power needed under full acceleration.
A larger engine, say a V6, has its benefits and weaknesses. But, each cylinder performs less work and the air-fuel mixture is also less, which naturally keeps knock at bay. See the complete explanation in the video right up above.
Comments
This is a very incomplete story here and some what misleading on todays engines. This is what feeds the urban myth of Turbo engines not being efficient.
#1 Compressions are not a low as you elude to. My LNF Ecotec was 9.5:1 compression.
#2 yes knock is a problem but what they are doing today to combat knock is pretty clever. First they went to oil squirting on the underside of the piston to cool the piston to lower combustion chamber temps. Next was the introduction of Direct Injection that not only placed the fuel precisely but also injects cool fuel from Injectors at several thousands of PSI.
These tricks drop the cylinder temps enough to raise the boost levels up to where the engine has good power at low off boost levels but adds much power at higher boost levels.
My LNF offered boost up to 16 PSI from the factory with Premium Recommended ratings. I added the GM tune and it went to Premium Required but it also sent the boost to 23 PSI on pump gas. It also added 555 HP to 295 HP and 315 FT LBS. The MPG went up 1-2 MPG.
I confirmed the fuel increase with Bill Duncan of GM performance and he said the added torque would get the vehicle up to speed faster and give it more off throttle time that cuts fuel consumption on Systems.
Even on my 3.6 the compression is now 11.5:1 due to the cooling oil and the DI injection. This is a regular fuel engine.
More work is being done with HCCI that will move compression to 18:1 where compression ignition will be used much like a Diesel on gas engines.
High boost and Compressions let smaller engines make more power but also run cleaner. This is why GM and other MFG are pushing for higher octane.
Very informative, thanks.
Do you know if liquid charge air coolers can also increase fuel economy since they lower intake air temps more than a traditional air intercooler?
I’m a sucker for click bait, but this one is so bad, I have to protest. Not only is it inaccurate, it appears that you ran out of ideas halfway through a paragraph.
Turbo fours don’t live up to economy expectations for one reason; People change their driving habits with small turbo engines, because of their narrow power bands. Turbo lag encourages people to keep the RPMs up in every gear. Even with an auto transmission, you have a tendency to hold the throttle open, and change your shift patterns.
It almost seems you automotive ‘journalists’ just started driving a couple of years ago.
Turbos have a wider power band.
That is not true for the turbo engines used in the average car.
https://www.quora.com/Why-do-automotive-enthusiasts-care-more-about-horsepower-than-torque-when-they-look-at-performance-cars
I agree the story is poorly done but use causation as today the turbo engines have very flat and powerful torque curves as the one here from the LNF.
Lag is really reduced with the higher compression and duel scrolled turbo units.
I do agree that the drivers when given more power will use it but even then they are still pretty easy on fuel.
Really, then what are the CAFE standards all about.
I’ll take a Big Block Chevrolet mated to the new 10 speed transmission… You can keep your turbo’s.
My Turbo 4 stock was 1 second faster than my two stock big blocks. The turbo may not have sounded as good but the torque and power was there.
I even turned on the traction control at 55 mph once, FWD sucks. The wastegate popped and I though I broke something.
I would rather have a V8 but I also accept the reality the opening is getting narrow on choices. Big Blocks are no longer a factory option.
matt, that big block might soon be more efficient as science is showing. you avoid wasted spent gas and turbo heat. For scott 3 the compression ratios on turbos is still high, but they vary them through intake valve timing. It might be a 10:1 ratio, but they let 10% of the air back out through the intake at open throttle so the actual air compression is 9:1, then you add you 16psi boost. Future motors will see HCC with DSF and variable compression as the leaders in price vs fuel economy. you can take GM’s current cam in block cheap design, and add homogenous compression plus variable intake timing and you have a engine that cost as much as todays engines, but will get at least 50% better fuel efficiency, if not more. Add mild hybrid and a Silverado could get 50mpg on gas.
The one thing you did not address is that the weight. Even in aluminum the engine still has a lot of mass and size.
In a time they are cutting mass anyway they can they will not add it to the engine.
GM has signaled were they are going in 2.7 ways.
Can supercar manufacturers really be wrong. I don’t think so.
Nissan GTR V6 twin turbo, McLaren V8 turbo, Ferrari V8 turbo, Subaru WRX boxer-4 turbo (threw that last one in – not a supercar i know – just proof of intense turbo-engined performance for decades).
I kind of understand why this clickbait is here. GM were not well known for their turbo engines until recently. NA V8s and NA V6s were (and probably still are) their bread and butter. But technology has moved on and so does GM. Diesel turbo, V6 turbos, the new Caddie 4.2L V8 turbo. Just waiting for that 5.5L V8 twin to materialise and make me grin madly.
GM was well known for turbo engines in 1962. Chevy and Oldsmobile.
My 2017 Chevy cruze gets 40mpg on the highway and still has quite fast acceleration
I have a 2017 Cruze, too. It’s my work car. I regularly average between 40 and 44 MPG, per tank full. While I’d love to have an Impala for work, because it’s far roomier, it’s hard to walk away from the awesome fuel economy of the Cruze!!
I think the GM 4-cyl D.I. turbos are a great package; I have an HHR LNF auto and a 2014 CTS LTG. But there is one area where GM needs to upgrade their turbo systems, the inter-cooler. I was driving in stop and go traffic on I-25 in CO recently, with the CTS, in road temps approaching 100F. Every time I would launch from a stand still it would go into full retard (my assessment w/o instrumentation) and torque would drop off dramatically and very slowly recover. I had trouble keeping up with traffic; it felt like it was a 2.0L N.A.! I believe with better packaging of the inter-cooler, away from the other system coolers, that the LTG would have operated satisfactorily in that high temp environment (obviously just my opinion). Also, a D.I. turbo package on CNG fuel (130RON) would have outperformed many of the other cars on the road that day.
I agree. In my ATS 2.0T the intercooler is placed between the A/C condensor and the radiator. Move the I/C to a position in front of these and you get better operation. I did this on my 2013 ATS turbo and also removed the factory I/C from between the radiator and the A/C condensor. Living in FL and the year round heat this allowed more advanced timing w/o knock.
All of this “talk” about gas engine efficiency is vague and pointless due to the burning of fuels, and the enormous loss of energy due to heat. Just replace all these engine setups with a hybrid power train (using the 5ET50 transaxle of the Chevy Volt) and you get quieter rides, cleaner running, longer lasting engines (with much more time between oil changes), and highest efficiency due to energy regeneration and storage during braking, then the reuse of that same energy (impossible with any internal combustion engine!!) to accelerate much faster with the electric motor.
Go hybrid or electric!! This is why a Tesla Model S can outperform ANY gas engine vehicle in the U.S.
Trump has all but dismantled the EPA , Scott Pruitt his henchman did a thorough job , too bad he spent a lot of the people’s money on himself and was removed as head . Now , California sets the table on emissions and have for years . Smog from the millions of vehicles got so bad that it mandated it’s own EPA policy simply because there were more vehicles and people there than any other State . If manufacturers wanted to sell vehicles there , they had to meet California’s new standards . Trump is trying to federally mandate this policy so California can’t continue to set their own policy . Sad part , as Trump doesn’t read , he has no clue about Smog or previous EPA concerns . It is all in his head , after a lifetime of experience , as he has said . What an amazing genius !
#MAGA
Laws of physics as you can’t get something from nothing and unless there’s a major breakthrough in technology, there’s just a maximum amount of power that can be generated from the type of fuel used; although increasing the fuel octane level may improve mileage.
You are correct about the max you can get but improved efficiency is where the gains are found.
Higher compressions and boost make the engine more efficient and the higher octane is needed to make the engine work.
Kind of like the Chicken and the egg. It is the Compression that makes it efficient but the octane makes it possible.
PARDON ME , I put my comment about dismantling the EPA under the wrong heading . First mistake I’ve made since I got married , and my son says ” the farmer hauled another load away and you could tell from the smell it wasn’t hay “
Lead footers are especially prone to see reduced MPG in a turbo. You’re asking a smaller liter engine to work harder with turbo boost. If you are a lead footed driver that means the boost of the turbo is happening longer and providing more boost than someone who squeezes the throttle. In some models the turbo engine is simply too small for the size and weight of vehicle. This creates a condition where the engine has to operate near max boost in order to provide a acceptable driving experience. If you look at how the EPA tests their ideal of highway speeds is around 45mph. Who does that these days? In other words it’s a series of tests that can give accurate A vs B comparisons but not real world results. The turbo boost just adds to the complexity and variables because its more sensitive to how you drive a turbo engine vs a naturally aspirated one.