1.8 Ecotec -- Extreme Fuel Economy

[dwightlooi]

The 1.8 Ecotec XFE is a concept centered around building the most most fuel efficient engine that will deliver 140hp -- the approximate baseline requirement for a FWD compact car in the same size class as the Cruze, Civic and Corolla.

Basic Design

• Type: Inline-3 w/counter rotating balance shaft
• Aspiration: Naturally Aspirated w/Part Time Atkinson Cycle operation
• Construction: Aluminum Block and Heads
• Displacement: 1788 cc (109 cu-in)
• Bore x Stroke: 88 x 98 mm
• Bore Center: 96 mm
• Valvetrain: Chain Driven SOHC 4-valve per cylinder (2 x concentric camshafts)
• Fuel Injection: Direct Gasoline Injection
• Compression Ratio: 11.2:1 (7.5:1 in Atkinson mode)
• Power Output: 140 hp @ 6200 rpm
• Torque Output: 130 lb-ft @ 4200 rpm
• Maximum Engine Speed: 6500 rpm
• Fuel Economy Est: 28 mpg (City) / 40 mpg (Hwy) w/6T40 6-speed automatic transmission

The engine implements five features to maximize fuel economy...

(1) Three Cylinders

The 1.8 XFE engine employs three cylinders instead of the more typical four. The bore, stroke and cylinder spacings are exactly the same as that utilized on the 2.4 liter Ecotec engines. Reducing the cylinder count reduces friction in the cylinder walls, in the crank case and in the valvetrain. This is because overall cylinder wall surface area, bearing surfaces in the bottom end as well as the valve train are diminished.

(2) Concentric SOHC

The engine employs a single cam sprocket and what can be characterized as a SOHC arrangment. This reduces the number of camshaft bearings and sprockets, again minizing frictional loses in the engine. However, unlike a traditional SOHC setup which prevents independent intake and exhaust valve timing control, the single camshaft in the XFE engine is actually two camshafts, one inside another. The cam sprocket drives the hollow outer intake cam via a hydraulic cam phase adjuster. The exhaust cam is inside the hollow intake cam and connected to it via a second cam phaser at the opposite end of the engine. The exhaust cam lobes slip on over the outer shaft and is pinned in place by with steel pins going through slots in the out camshaft. This is similar to the cam-in-cam setup in the Dodge Viper's Pushrod 8.4 liter V10.

(3) Part Time Atkinson Cycle

The intake valves feature a 2-stage cam profile switch system. Not unlike Honda's VTEC or Toyota's VVTL-i there are two cam lobes for each set of intake valves. The valves' rocker arms normally follow one set of lobes, whereas an idler rocker follows the second set. At cruising conditions and at low acceleration loads, the engine switches over to the second set of intake cam lobes. These cam lobes implement Atkinson Cycle operation. In essence, they keep the intake valves open long into the compression stroke. Air is allowed to escape back into the intake manifold for the first 33% of the compression stroke. This in turn reduces the effective displacement and compression of the engine. The expansion stroke however remains the same 98mm in length which maximizes energy recovery from the fuel burned. Traditionally, Atkinson Cycle engines (employed in the Prius and the Fusion Hybrid) sacrifices performance for improvements in brake specific fuel consumption. However, since this engine switches between the normal Otto Cycle and the Atkinson cycle as needed, no performance is sacrificed.

(4) Direct Gasoline Injection

Gasoline is injected directly into the cylinders. With direct injection, gasoline is introduced into the cylinders during the compression stroke. This prevents detonation and knocking prior the injection of fuel. It also produces a cooling effect from the atomization of the fuel. This permits the engine to operate using a higher compression ratio increasing thermal efficiency.

(5) Stop on TDC

The XFE's ECU stops the engine when the vehicle slows to a halt for more than 3 seconds. Unlike previous GM idle-stop implementions, there is no starter motor or BAS involvement in restarting the engine. Instead, the ECU (through a high precision crank position sensor) stops the engine when one of the pistons is just past Top Dead Center after its compression stroke. To restart, the ECU simply utilizes the direct injection system to introduce fuel and fire the the spark.

Edited July 24, 2010 by Oldsmoboi

[Drew Dowdell]

What about HCCI instead of Miller cycle?

[dwightlooi]

What about HCCI instead of Miller cycle?

I don't think GM knows how to do it right just yet. HCCI requires the cam switching, it also requires real time cylinder pressure monitoring. Atkinson cycle cannot damage the engine, HCCI can if managed in correctly or if the management sensors fail over time.

BTW, there is a difference between Miller and Atkinson cycles. Yes, they both keep the intake valve open into the compression stroke. But, Miller Cycle does not kick the intake charge back into the intake plenum, it continues to fill the cylinder as the piston is going up, closing the intake valve when airflow approaches the inversion point but not after. Miller engines do this by using a supercharger to maintain positive pressures in the intake plenum, so air continues to flow into the cylinders even as the piston is going up. There is hence a difference in intent between the two. Atkinson cycle strives to achieve an asymmetrical compression and expansion stroke -- the latter being longer than the former for greater energy recovery. Miller cycle strives to achieve more complete cylinder filling by giving the cylinders more time to fill than in a traditional supercharged powerplant.

Edited July 24, 2010 by dwightlooi

[Drew Dowdell]

ok. I understood the differences. I think you mis-typed something in your original post then. I thought you were going to for part time miller cycle somehow using a turbo

[dwightlooi]

ok. I understood the differences. I think you mis-typed something in your original post then. I thought you were going to for part time miller cycle somehow using a turbo

Yes, I did. I meant to say Atkinson cycle, but the time for limit for editing a post had expired when I realized the mistake.

[Drew Dowdell]

fixed it for you

[dwightlooi]

fixed it for you

Thank you. Such attentiveness and considerations are, to say the least, uncommon in an internet forum.

[Drew Dowdell]

That said, I think you'd have a hard time selling this engine in the US in anything other than the Aveo and Spark.

[smk4565]

A 3-cylinder is a hard sell, unless as a generator in the Volt or something like that. Why not just sell a diesel, they could get 50 mpg from a 4-cylinder diesel and it wouldn't put out a measly 130 lb-ft. I think selling a 160 hp, 50 mpg diesel is easier than selling a 140 hp, 40 mpg 3-cylinder gas engine. Or make more hybrids. But many of these carmakers are just coming out with new gas engines, trying to get an extra 1-2 mpg out of it, which is nice, but to make a splash you need to be at 50 mpg, not 40. The Prius was over 50 years ago, a VW Polo is around 60 mpg. To me, 3-cylinder says Smart For2, tiny, painfully slow car, so that isn't at all appealing. Now if someone builds a car that is 0-60 in under 8 seconds, but getting 40+ mpg, then they are on to something (Sonata hybrid might do that).

[regfootball]

most folks would say why not just a typical 4 cylinder, why change what already works? Spend the time working on enhancing the design of the parts already there, the torque band, and the gearing. spend the money on an efficient and RELIABLE (domestic mfrs thats you) CVT which would be perfect application for this kind of car.

explain why 3 cylinders and only 12 valves as opposed to 4 combustion chambers and 16 valves (more valve area) would allow more air in and out and more power?

Or is this just an exercise to save manufacturing parts, but charge the same price for something that has less power? that's what it sounds like. 3 cylinders = 3/4 the price. 1 cam = half the price.

If you want fuel economy, oversize a nice proven four with DI and mate it to a CVT and spread the gearing wide.

the challenge with a small motor is to keep it operating in the optimal torque band and rpm so as not to have so much rpm.

i thought you said small fours didn't need balance shafts, why do a 3 with a balance shaft if you can have a 4 without.

would this mythical 3 be well suited for turbo charging?

Edited July 25, 2010 by regfootball

[dwightlooi]

explain why 3 cylinders and only 12 valves as opposed to 4 combustion chambers and 16 valves (more valve area) would allow more air in and out and more power?

Actually, valve area has nothing to do with the total umber of valves or the number of cylinders. With 3-cylinders you will still have 4-valves per cylinder and you can have the same ratio of valve area to bore area if you scale everything proportionally.

The advantages of less cylinders is reduced frictional loses. The same advantage you get in say a 2.4 liter four cylinder vs a 2.4 liter 6-cylinder. The 6-cylinder has more cam bearings, more cam lobes, etc. In addition, if you do the math, a cylinder bore's surface area (the area the piston rubs against) is smaller if there are less cylinders for the same displacement. The bore area is determined by the circumference. If you take two circles, one with twice the surface area of the other, you will notice that the circumference of the larger circle is not twice as large even though the area is twice as large. This is because Area is a function of the Square of radius, whereas the Circumference is a function of 2 x Radius.

In short, for any given displacement, friction is reduce if the cylinder count is reduced.

Or is this just an exercise to save manufacturing parts, but charge the same price for something that has less power? that's what it sounds like. 3 cylinders = 3/4 the price. 1 cam = half the price.

If you want fuel economy, oversize a nice proven four with DI and mate it to a CVT and spread the gearing wide.

the challenge with a small motor is to keep it operating in the optimal torque band and rpm so as not to have so much rpm.

i thought you said small fours didn't need balance shafts, why do a 3 with a balance shaft if you can have a 4 without.

would this mythical 3 be well suited for turbo charging?

The reduced complexity does reduce cost as a side benefit, but the merits of the configuration is substantial even when costs are equal. The idea is not to have a smaller engine. The engine is still 1.8 liters -- same displacement as the Civic, Focus, Corolla or Cruze four cylinders -- it simply has one less cylinder, one less cam and the added benefit of part time Atkinson Cycle operation as well as DI.

Same displacement, less frictional loses = more torque and more power (compared to a 4-cylinder 1.8 liter)

A big 3-cylinder with a balancer is similar in vibration levels to a small 4-potter without one. Which means vibrational harshness should be similar to the competing engines.

As far as turbo charging goes, a 3-cylinder actually as one advantage. It doesn't need or care for a twin scroll turbo. The purpose behind a twin scroll turbo and the segregated exhaust manifold that feeds it centers around a unique problem with turbo fours. In a 4-cylinder, the pistons are 180 degree apart in position. This means that the very moment a cylinder is at the beginning of the exhaust stroke (bottom of the power stroke travel), another piston is inconveniently at the top of the travel and in the overlap period where both the exhaust and intake valves are open. The high pressure exhaust pulse from the cylinder beginning its exhaust stroke feeds back into the cylinder at the end of its exhaust phase preventing proper scavenging. Traditionally, this is why turbocharged engines (without twin scroll turbos) do not like overlaps at all and simply prefer to live with the inefficiencies of closing the exhaust valve early. This is also why twin scroll turbos exist. They along with their manifolds separate the exhaust flows of cylinders 1 & 4 from that of 2 & 3 so that exhaust pulses do not interfere with optimal scavenging. With a 3-cylinder engine, this problem simply does not exist. The cylinders are 120 degrees apart; as one cylinder is at bottom dead center neiter of the other two are any where near Top dead Center. Hence, 3-potters can use a single scroll turbo and proper overlaps with no penalties.

[regfootball]

i know overseas we see a lot of smaller 3 poppers. the obvious question then is why this approach is not popular on an engine like a 1.8.

Edited July 25, 2010 by regfootball

[dwightlooi]

i know overseas we see a lot of smaller 3 poppers. the obvious question then is why this approach is not popular on an engine like a 1.8.

Traditionally, 3-potters shake more than 4-potters because it has 1st order imbalance vs the 4-potter's 2nd order imbalance. Perceived degradation in civility is almost always worse with 1st order imbalance*. This is compounded when you use fewer, larger cylinders. Also, with 2nd order imbalance like you see in 4-cylinder engines, vibration gets really bad only at higher rpms, with 1st order imbalance it is noticeably bad from idle and up. At 1.0~1.3 liters 3-potters are OK, go larger and they become significantly unrefined. However, that is true only when we are talking about engines without balancers.

With a single balancer, a 3-potter can tame most of its 1st order imbalance putting it on a similar vibrational footing as a 4-potter without balance shafts. With two balance shafts a 4-potter is even better.

But, as we can see, the market is full on unbalanced 4-potters -- all GM 4-potters smaller than the 2.0 DI Turbo are unbalanced, the Ford 2.0 Duratec is unbalanced, the Honda Civic's R18A is unbalanced, the Corolla's 1.8 1ZZ-FE is unbalanced. In general, you'll see that sub 2 liter 4-potters are generally unbalanced. The ONLY exception I know of in recent years is the 1.8 liter Kompressor engine in the C230 Kompressor which has balancers. About a 2/3rds of 2 liter mills aren't either. Most 4-cylinder engines in the 2.4~2.5 liter class are balanced. Hence, being on the same level of vibrational harshness as unbalanced 1.8s is not particularly horrible.

* A simple way to understand the differences between 1st order and 2nd order vibrations is that 1st order vibrations are from a constantly shifting center of gravity, 2nd order vibrations on the other hand is from the differences in acceleration between pistons near the top and bottom of their strokes. 1st order vibrations has a frequency of the engine's current rpm, 2nd order vibrations occur at twice the frequency of the engine's rotating speed. 1st order vibrations in a 3-potter (or a V6) is tamed with a single balancer shaft turning in the opposite direction but at the same speed as the crankshaft. 2nd order vibrations must be tamed with two balance shafts turning in contra-rotation to each other at twice the speed of the crank shaft.

[CanadianBacon94]

hm.. the turbo 660 in my A-Cat runs really smooth.

but i guess that would count as a very small 3.

[regfootball]

Traditionally, 3-potters shake more than 4-potters because it has 1st order imbalance vs the 4-potter's 2nd order imbalance. Perceived degradation in civility is almost always worse with 1st order imbalance*. This is compounded when you use fewer, larger cylinders. Also, with 2nd order imbalance like you see in 4-cylinder engines, vibration gets really bad only at higher rpms, with 1st order imbalance it is noticeably bad from idle and up. At 1.0~1.3 liters 3-potters are OK, go larger and they become significantly unrefined. However, that is true only when we are talking about engines without balancers.

With a single balancer, a 3-potter can tame most of its 1st order imbalance putting it on a similar vibrational footing as a 4-potter without balance shafts. With two balance shafts a 4-potter is even better.

But, as we can see, the market is full on unbalanced 4-potters -- all GM 4-potters smaller than the 2.0 DI Turbo are unbalanced, the Ford 2.0 Duratec is unbalanced, the Honda Civic's R18A is unbalanced, the Corolla's 1.8 1ZZ-FE is unbalanced. In general, you'll see that sub 2 liter 4-potters are generally unbalanced. The ONLY exception I know of in recent years is the 1.8 liter Kompressor engine in the C230 Kompressor which has balancers. About a 2/3rds of 2 liter mills aren't either. Most 4-cylinder engines in the 2.4~2.5 liter class are balanced. Hence, being on the same level of vibrational harshness as unbalanced 1.8s is not particularly horrible.

* A simple way to understand the differences between 1st order and 2nd order vibrations is that 1st order vibrations are from a constantly shifting center of gravity, 2nd order vibrations on the other hand is from the differences in acceleration between pistons near the top and bottom of their strokes. 1st order vibrations has a frequency of the engine's current rpm, 2nd order vibrations occur at twice the frequency of the engine's rotating speed. 1st order vibrations in a 3-potter (or a V6) is tamed with a single balancer shaft turning in the opposite direction but at the same speed as the crankshaft. 2nd order vibrations must be tamed with two balance shafts turning in contra-rotation to each other at twice the speed of the crank shaft.

if you are tuning this engine for fuel economy, you're not going to be as obsessed about its high rpm behavior. you'd be more concerned with its vibation levels at lower rpms, where you want the engine to be more often than not, to keep fuel consumption down.

[the_yellow_dart]

Part-time Atkinson - has that ever been done before?

One thing that strikes me as interesting too is restarting with the pistons - has any car gone without a starter motor before??

[Drew Dowdell]

Part-time Atkinson - has that ever been done before?

One thing that strikes me as interesting too is restarting with the pistons - has any car gone without a starter motor before??

Rolls did it in the 20's or 30's. But I think that was also a straight 8.

[regfootball]

one thing i have wondered. GM had BAS, where it shut off the engine while at stop. couldn't an engine just be made to turn off say 2 of 4, or 3 of 6 cylinders while a car is idling? Kind of like extending cylinder deactivation to when an engine is idling.

then you would in theory cut fuel consumption at idle, and you wouldn't be turning the engine on and off, and having all the wear......

Edited July 27, 2010 by regfootball

[daves87rs]

Aveo or Spark would work.

Running on 3 could work, though my question is abuse that it could take....

[dwightlooi]

Part-time Atkinson - has that ever been done before?

One thing that strikes me as interesting too is restarting with the pistons - has any car gone without a starter motor before??

(1) Part Time Atkinson hasn't been done before on production vehicles to the best of my knowledge. Atkinson Cycle and Cam Profile Switching, however, has been done to death.

(2) Restarting using pistons is currently done by Mazda on the Mazda 3. They call it i-Stop.

http://www.mazda.com/mazdaspirit/env/engine/siss2.html

[dwightlooi]

Rolls did it in the 20's or 30's. But I think that was also a straight 8.

If I remembered correctly, that was a pneumatic start system. Quite a bit different as it involved using compressed air in the intake to move the pistons. Actually, many really BIG engines such as the ones you find in tanks are pneumatic start.

Edited July 27, 2010 by dwightlooi