Toyota's astonishing new Atkinson Cycle engine delivers 78 mpg


April 13, 2014

In order to achieve significant improvements in thermal efficiency, the Atkinson Cycle engines have been lavished with numerous innovations including a reshaped intake port, increased compression ratio, idling-stop, Variable Valve Timing intelligent Electric (VVT-iE) and a cooled Exhaust Gas Recirculation system (highlighted in image)

In order to achieve significant improvements in thermal efficiency, the Atkinson Cycle engines have been lavished with numerous innovations including a reshaped intake port, increased compression ratio, idling-stop, Variable Valve Timing intelligent Electric (VVT-iE) and a cooled Exhaust Gas Recirculation system (highlighted in image)

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Toyota has announced the development of two hyper-fuel-efficient small-displacement petrol Atkinson cycle engines: a three-cylinder 1.0-liter and four-cylinder 1.3-liter which will be introduced across the range from next year in 14 different variations. The smaller engine will deliver 78 mpg (US) in the Toyota Aygo, an improvement of 30 percent.

The fuel-efficiency of the one-liter engine in particular is remarkable. The engine it will replace won the the One liter category of the International Engine of the Year awards four years in a row from 2007-2010.

At this year's Geneva Motor Show just a month ago, a revised version of the award-winning three-cylinder 1.0-liter engine was announced as the powerplant for the new Aygo.

Numerous tweaks were used on the reengineered engine such as a higher 11.5:1 compression ratio, an improved combustion chamber design, reduced frictional losses and a weight reduction courtesy of a cylinder head with a built-in exhaust manifold.

With all these changes, the unit's fuel economy had been reduced to 3.9 l/100 km (60 mpg US) at the same time as more power (68bhp – 51kW @ 6,000rpm) and more torque (95Nm @ 4,300rpm).

Now the new improved unit will be replaced again with an Atkinson Cycle engine of the same three-cylinder, 1.0-liter layout, which it has jointly developed with Daihatsu.

Though Toyota has previously used Atkinson cycle engines in its hybrids, it's the first time that the Atkinson design will be used as a stand-alone unit, and Atkinson Cycle engines aren't known for their low- and mid-range torque, but Toyota claims it has addressed this issue with a number of innovations.

These innovations include a reshaped intake port designed to generate a strong tumble flow (whereby the air-fuel mixture flows in a vertical swirl) inside the cylinder, and a cooled Exhaust Gas Recirculation (EGR) system, Toyota's Variable Valve Timing intelligent Electric (VVT-iE) technology, an idling-stop function, high compression ratio and various unnamed fuel consumption reduction technologies.

The end result is a maximum thermal efficiency of 37 percent and a fuel efficiency improvement of "approximately 30 percent over current vehicles" – that's the difference between 60 mpg (US) and 78 mpg (US) and it's an astonishing improvement.

Toyota Aygo owners prize the car for its performance at the petrol pump, not the traffic light Grand Prix, and the Aygo is largely purchased for use in tight, congested, urban roads. The appeal of the Aygo and the other compact Toyota vehicles which will use the unit will be greatly increased.

The Japanese manufacturer plans to introduce 14 variations of the 1.0-liter three-cylinder and its 1.3-liter four-cylinder bigger brother by 2015.

The larger 1.3-liter Atkinson cycle engine uses a high compression ratio (13.5) and all the same engine innovations to achieve thermal efficiency of 38 percent. The efficiency results in the 1.3-liter engine aren't quite as spectacular as the one-liter engine but fuel efficiency gains of approximately 15 percent have been realized by comparison with current vehicles.

About the Author
Mike Hanlon Mike grew up thinking he would become a mathematician, accidentally started motorcycle racing, got a job writing road tests for a motorcycle magazine while at university, and became a writer. As a travelling photojournalist during his early career, his work was published in a dozen languages across 20+ countries. He went on to edit or manage over 50 print publications, with target audiences ranging from pensioners to plumbers, many different sports, many car and motorcycle magazines, with many more in the fields of communication - narrow subject magazines on topics such as advertising, marketing, visual communications, design, presentation and direct marketing. Then came the internet and Mike managed internet projects for Australia's largest multimedia company, (Australia's largest Telco), (Australia's largest employment site),,, and a dozen other internet start-ups before founding Gizmag in 2002. Now he writes and thinks. All articles by Mike Hanlon

@LowFlyerXX: "Although, gas is more expensive - about a third more than ave US."

Big error! Gas is about 3X times more expensive in Europe.


I wish this article had briefly explained:

1) what the Atkinson cycle is 2) what the effect of cooling the exhaust is

Mark Robinson

I wonder how Toyota's 1.0L 3-cylinder will compare with Ford's EcoBoost I-3? While the Ford isn't as efficient, it has more HP and more torque. Also, the Ford I-3 is designed to use 87-octane fuel. Is the Toyota?

Paul Stregevsky

stick the 1.0L engine in the Prius.

Tom Swift

LOL, you'll need a BIGGER torque engine, or with 2-3 people in the vehicle, you'll never be able to climb a slight incline on an interstate highway without getting out to push.

Rusty Harris

Compare progress of automobile engines with aircraft jet engines. Since around 1940 jet engines have gone from less than 1,000 lb. thrust to 100,000 lb. Power to weight ratios have increased enormously, specific fuel consumption has dropped dramatically and reliability has soared. You have to wonder what the auto engine people have been doing all their lives - just little improvements over multi-decades. I appreciate that jet engines designers have bigger budgets, but really ..............


@Rusty Harris laughing at small Japanese engine design is part of what was responsible for the downfall of Detroit. Have you seen Detroit lately?

It's not that they aren't able to make more power with a 1000cc engine, its that they aren't able to do it as efficiently. 1000cc motorcycles make close to 200hp now but they aren't designed for fuel efficiency.

My first car was a manual 1989 Ford Escort that only made 90 HP and it still had more oomph than was legal to use on public roads. I think it only takes 10 to 20 HP to maintain highway speed so even then there is enough power left for a slight incline or 2. You rarely actually use the full power of most engines and even when you do its only for a few seconds at a time.

For the rare cases you do need the extra kick it's been proven more fuel efficiency to use a hybrid electric design than to throw a large displacement engine at the problem that serves as overkill 99% of the time.


VW has sold the Polo for years with a clean diesel that gets in the 70's range of mpg. They won't sell it in the US.


It would have been interesting to learn what the Atkinson Cycle is, instead of quoting press release data.

Erik Unger

30% is a big improvement, and 68bhp is tons for a small car. @Rusty - I've suffered the indignity of a car not having enough go to get up a hill, but that was a 3.5litre V6 Mustang coming out of Yosemite... Heap of junk that was!


To be pedantic, the engine does not have an extremely high compression ratio because the inlet valve stays open at the start of the compression stroke and only closes when a significant portion of the fuel charge has been pumped back into the inlet manifold. This reduces the final compressed pressure below that which would cause pre-ignition or detonation. The compression ratio is what is left of the compression stroke in relation to the cylinder-head volume. The expansion stroke is not affected and can thus legitimately be described as 'extremely high' when the cylinder-head volume is compared to the expanded volume measured from t.d.c. to b.d.c. This expansion ratio, when placed in the 'r' term in the formula for thermal efficiency instead of that for the compression ratio leads to the improved fuel economy figures. I imagine they need a fly-by-wire throttle to manage the ram effects around the point of valve closure.

I personally know of a certain British motor manufacturer that could have had this in development as long ago as the mid-eighties, but it relied on having a management that was worthy of the term.

It is a matter of conjecture as to whether the size of these engines should be that of the b.d.c. to t.d.c. value, as at present, or should be that of the actual compressed volume that remains after closure of the inlet valve, which will surely be important when taxation, insurance and such like are taken into consideration. That assumes, of course, that one can fix an actual value for a variable valve timing system.

Mel Tisdale

nothing is mentioned about it's exhaust...will it be approved for california for example? also nothing is mentioned about repairs, i.e. easy to repair?, how long does the engine last?, expensive parts? Is it louder or quieter than the usual engine? a 1000cc engine seems to be the standard now days for a UTV...will it withstand shocks?, cold weather, cold water splashing on it when it is hot? Can it be modified for small airplane use? How about use in boats? Motorcycles? Speaking of sister had a smaller Harley back in the days of AMC and the plastic parts of the carb broke due to vibration. you couldn't buy the parts, you had to buy the whole carb. so my bro-in-law bought a standard carb from NAPA and fitted it on. It got 78 mpg and had more power! Who would have thought a carb could make so much difference?


@Diachi - Quote: "You rarely actually use the full power of most engines and even when you do its only for a few seconds at a time."

This is why a properly designed hybrid gets stupid good milage. The ICE engine is sized for average load not peak load. The battery or hydraulic acumulator makes up for the peaks and charges during braking and between peaks. This is also why most hybrids foisted on the public by traditional mindsets like Rusty's get only marginally more power. They are thinking of the battery only as a recovery tool, not as a reservior. The first time I saw a hybrid car it was an excersize in cross disapline research by a school. They had the hydraulics engineers build a car for mileage. The students put a 10HP engine in the back running at 3/4 open all the time and a hydraulic accumulator with a reversable pump/motor as the prime mover. It had regenerative braking and got 80mpg while pulling 0-60 in 5.5s. Unfortunately the traditional companies looked at it and claimed it was impossible, despite the working prototype, and missed the concept.

This is why vehicles like the volt will never get great milage and cost far more than needed. When you buy a Volt you are buying two cars not one. You are buying an electric vehicle with a range of 20-30 miles and an ICE vehicle that only gets 30mpg. If they had rigged it the same way the students did they could have used a battery half the size and a 25hp diesel engine while selling for a 10% markup and giving a much better ROI.


It makes me mad, all these "attempts" (and as off lately Governmental push) to lower gas consumption - as VoiceOfReason said - Europe (not only VW) has had cars that get well over 70 MPG!! Althouhg, gas is more expensive - about a third more than ave US. And unfortunately, back in late 50's Ford did well over 100 MPG with experimental carb from Rochester fitted on a Flathead motor - I have this from a trusted source... Makes me wonder if it is all another form of political "muscle flex", every gallon sold = taxes to the state and federal!! OUT of our pockets. HHHMMMMM... But if I choose to ride a big comfortable and powerful SUV - do I suffer the extra added cost for fueling it? I have a choice, or do I?? Open for debate...


Slick 50, full synthetic oil, ionized air intake and fuel, K&N air filter, all current technologies readily available makes an engine much more efficient. All about lowering friction and more efficient combustion burn. Now combine that with the new Toyota 1.3 liter 4 cylinder, and you may have the ultimate commute car engine when combined with a safe and light chassis. Not to mention a possible SCCA box stock racing champ.

Perry Obray

Mark Robinson 14th April, 2014 @ 08:28 am PD

"...I wish this article had briefly explained:

1) what the Atkinson cycle is

2) what the effect of cooling the exhaust is Mark Robinson 14th April, 2014 @ 08:28 am PDT ..."

Yeah, me too, but Google means the information is only 1 second away! And I note that it is not something you can summarize in one sentence.

"...The original Atkinson cycle piston engine allowed the intake, compression, power, and exhaust strokes of the four-stroke cycle to occur in a single turn of the crankshaft and was designed to avoid infringing certain patents covering Otto cycle engines.[1] Due to the unique crankshaft design of the Atkinson, its expansion ratio can differ from its compression ratio and, with a power stroke longer than its compression stroke, the engine can achieve greater thermal efficiency than a traditional piston engine.

While Atkinson's original design is no more than a historical curiosity, many modern engines use unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke, thus realizing the fuel economy improvements the Atkinson cycle can provide.

The ideal Atkinson cycle consists of following operations:

1-2 Isentropic or reversible adiabatic compression 2-3 Isochoric heating (Qp) 3-4 Isobaric heating (Qp') 4-5 Isentropic expansion 5-6 Isochoric cooling (Qo) 6-1 Isobaric cooling (Qo')




@Rusty Harris Laugh at Toyota if you wish but the last time I drove a car that struggled to climb a moderate grade it was a Chrysler PT Cruiser rental on a drive from Phoenix to Flagstaff. I could barely maintain 50 while everything from big rigs, to Yarises and Aveos blew past us.

This slug wasn't even my first choice; I had requested a Kia subcompact but was stuck with the PT Looser leftover.


Want some really crazy history of this engine? Go to...


I thought Atkinson cycle engines were confined to use in hybrids and other constant speed applications.Running it at variable speeds cuts fuel economy. I think the ICE will never be as clean burning as an external combustion engine-you will always need exhaust scrubbers,because the fuel can never be completely burned in the milliseconds it is in the combustion chamber. ECEs like the one being developed by Cyclone Power Technologies are poised to replace the ICE with an efficient engine that burns anything combustible with complete combustion:


All I want to know is how much the 1.0 weighs and can I replace it myself when it goes bad?


Rusty, cars of this type are not designed for interstate/long distance trips.

Noel K Frothingham

I drive a ONE liter Geo Metro 4 door hatchback that is a 1991 model year. I live in San Diego and go to El Centro often in hunting season to hunt dove, quail, pheasant. This trip is about 120 miles each way and the grade is very steep both going and returning. Obviously I climb the hill and come down the other side in both directions. I get over 50 + MPG. In town this car averages 38 MPG. I keep very good records of mileage and gas purchased. This car is over 23 years old. It pulls the hills fine at 55 miles an hour and takes very minimum maintenance. It is not a dragster but does not lack for passing power. Years ago had a Chevy ll which would not even pull a moderate hill without revving and popping into gear. The Geo has a one liter engine from a former Suzuki airplane motor. If they could produce this in 1991 what has happened since? The oil conglomerates, that's what.


Totally impressive, but real world figures will be the proof. This does make one wonder about the next hybrids - will they be competitive?

But: I have read that these high compression engines have higher particulate emissions - NOT good.

Simon Gray

@Virtual Gathis: You are misinformed about the Volt. Its electric range is 38 miles (according to the EPA and owner experience), not "20-30" as you claimed. It gets 37 MPG while running on gasoline (again, according to the EPA and owner experience), not "30" as you claimed. The gasoline engine in the Volt is limited to only 90 HP, which is not enough for peak demand, only average demand. So the battery is, in fact, used as a 'reservoir.' Drivers get spoiled by the silence of electric propulsion, so any engine noise becomes annoying. The noise, vibration, and harshness from a 25 HP engine screaming away all day at high RPMs would be unacceptable to most consumers (as would the limitations on its performance up mountain passes). * Finally, the fleet average (per GM) is 63% of Volt miles on electricity. Mine is more like 75%, so I would not be willing to shell out the extra cost for a highly-efficient (Atkinson or Diesel) engine that I only use occasionally.

Bob Garlough

@geoffg "Compare progress of automobile engines with aircraft jet engines. Since around 1940 jet engines have gone from less than 1,000 lb. thrust to 100,000 lb. " there has been a 100 fold increase of power in about 60 years of progress Considering that that by the end of the second world war aircraft engines were making in excess of 2000 hp I would contend that there was over a 100 fold increase in power in approximately the same amount if time. I wold also content that in terms of reliability and efficiency. Now also consider that the technology available in 1940 was vastly superior to that in the 1880's development internal combustion engine could have been much slower. I guess you were expecting the we would have flying cars along the lines of George Jetson by now.


Just for information Maruti-Suzuki has been selling 800 cc cars for more than 15 years. It can comfortably cling long hills at an incline of 30 degrees with a load of more than 300 Kgs. This too in an ambient temperature above 90 degrees. Only reason that justifies buying a big car in the US is the fact that a small car DOES get sucked into the vacuum created by 18 wheeler tractor trailers going at 80 mph on highways or like the car drifting across 2-3 lanes in Chicago - the windy city - when it really gets bad. I have witnessed both phenomenon personally.

Nowadays I am pretty happy with 1.1 liter Hyundai i10 which carries 3 of my family members weighing around 250 Kgs in total.


@ LowFlyerXX The flathead with the trick carb story has never added up to get the alleged mileage you have to make the fuel air mix to thin to burn.

@ Noel K Frothingham If the car is not capable of it I certainly won't buy it. If the car can't do a road trip it a silly toy.


I have got to own up! When I was being pedantic in my original comment, I was working on the notion that this was a late intake valve closure design, and my comment was in relation to that.

Just goes to show that you are never too old to learn something new, and never too old to think that you know it all! Mea culpa, sorry!

Mel Tisdale

Can anyone tell if this a true Atkinson with longer expansion stroke, or a modified-Otto with delayed closure of intake valves as commonly used in hybrids?


yawn. They've been doing this with diesels for years. This is not a true atkinson cycle, but modified. The original atkinson cycle used lower compression and what I think is a longer duration for closing the intake valve.

i'd love to see an engine like this saddled with a kers style diff as seen in the volvos, but placed in a small, light weight city car. You could still get good acceleration, have a fun, nimble vehicle and still get good fuel economy. Nearly the best of both worlds!

@ tigerprincess Safety is what killed the geo metro. Sure they got 40+mpg years ago, but the body was/is a tin can. Increasing government safety requirements have increased the weight of the car, which is the enemy of fuel efficiency. I too had one of the 3cyl 5spd MT models and it was very fuel efficient, but it did not feel very safe. I"m sure some youtube crash videos would back up my claim.

Michael Wilson

what a lovely engine. how can anybody say anything bad about this marble of an engine. 37% efficiency! sell this thing in the State.. I will buy one.. I am sure it will be cheap, right? Yaris price?


My Prius uses the modified Atkinson cycle to about the same thermal efficiency , but it is combined with the regenerating electric drive which replaces the total loss conventional transmission these cars will also need. Not only that, the Prius keeps its peak revs below 5400 rpm. That saves the sort of engine wear that these tiny power plants will undoubtedly have when they scream to 8000 rpm or more getting onto freeways. That is, I think, what Toyota has in mind. they have heard from their unhappy dealers that Prius' have less need for repair service, greater longevity, and longer lasting brakes, taking some of their profits from service and replacement sales away. It is not really progress to keep making cheap disposable cars.

Paul Gracey

Atkinson engines of this sort aren't true Atkinsons but are a derivation. I don't particularly care for this but I love the fact that they are constantly innovating.

Hardly worth comparing turbine engine technology with ICE technology. The designs are sooo different from the outset...

I had to rent a Prius, just to see what the roar was about. I rented it for a 4 day weekend and drove it about 700 miles. I could not stand it and I achieved, near as makes no difference 49 mpg. Heck a Honda Civic will get you 45 mpg or more if you are careful at ⅔ the price.

I am not just a Prius hater across the board but my summation looks like this (because I know so many of you care!) Cramped cockpit for driver, 3 children in the back cramped, wife in passenger seat cramped. I am military and 6 foot 185 lbs., my wife will kill me if I mention that she weighs 155 lbs. so I will leave that out, children 10, 14 & 16 of 60, 105 & 120 lbs respectively. Adequate storage trumk Beauty is in the eye of the buyer, I am not so it is UGLY. Despite electric torque off the line the damn things creep along after a green light.

Otherwise okay...I guess.

Dr. Veritas

Whoopy-do. So with all this engineering and expense we're still going to end up with a car that basically throws away fully 60% of the petrol that we put in the tank. Roll on the EV revolution - smooth, quite rides, grow your own fuel on your roof and, best of all, 2 fingers up to Big Oil and all their ghastly cronies. MW

Martin Winlow

If the engine performs as described (ie 78 miles per gallon US or 94 mpg for UK, say about 75 mpg for real world UK) then surely this spells the end for diesel and hybrid for cars and light vehicles. What's the point of the additional expense and complication of these systems? The new Toyota engine seems to have port injection so will be cheaper and inherently clean. Be interesting to see how it performs in the real world. Could be a game changer.

Dibble Dobble

A 30% improvement in fuel efficiency compared to existing engines is unrealistic. Expect maybe 10%.

Dibble Dobble

For those calling the cycle "modified atkinson" - you're right - it's a Miller cycle.

The only way to get low and midrange torque back on these engines is to use a low-boost turbo or suprercharger (or both) and it would have to be linked to a CVT in order to maintain optimum RPM in any case.

Toyota have a lot of experience with this mode of operation. Their gas turbine vehicle developments in the 1970-80s addressed the issue of long reaction times between throttle change and engine speed by keeping the engine speed constant and taking advantage of the fact that available torque changed instantly with throttle changes.

As for noise: The majority of exhaust noise is a "puff" of gas escaping as the valve opens. Because the exhaust stroke is proportionately longer than the inlet, more energy is extracted from the stroke and chamber pressure is lower when the valve opens - so yes, they are quieter.

Interestingly in modern engines a good chunk of the noise is actually induction noise and that is addressed by running longer inlet tracts - but that in turn means that inlet losses need addressing.

Frictionally: the largest source of friction in an engine is the valvetrain. Cam-based poppet-valve systems are mechanically simple but they will have to go, long term and be replaced with springless electronic or hydraulic actuators (I am surprised that desmodronic valves hasn't made a reappearance in cars yet)

What's been getting the most gains in friction recently is paying more attention to finish levels, with modern pistons, bores and other bearing surfaces being mirror-finishes and often surface-hardened too.

I do find it interesting that for all the hype in the 1990s that ceramic engine components were the way of the future (they allow engines to run hotter, which improves thermal efficiency), nothing meaningful has emerged along these lines in the last 20 years - this has a lot to do with the issue that hotter combustion means higher NOx emissions if not carefully regulated.

On the subject of NOx emissions: Only Euro4-5 engines meet USA NOx emission standards. Older Eurodiesels simply can't meet those standards, as they were never designed to control NOx emissions until quite recently. Coupled with USA diesel being pretty filthy until about 2001 (extremely high sulfur levels), it was more a case of european makers not being allowed to sell their engines into the USA than not wanting to.

Of course, as others have pointed out, external combustion has a number of advantages - but whilst in a stationary configuration an IC engine putting its output heat into a Stirling engine might be worthwhile, the weight penalties in a small car make it a pretty dubious proposition and standalone ECEs suffer from warmup time penalties before they can be used (that's why steam engines fell out of favour), plus can't be directly coupled to the roadwheels so you'd have to make it a hybrid setup - with weight penalties.

It will be interesting to see where Toyota go with these new engines....


I wish they'd stop making interference engines.

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