Variable Compression - Saab SVC

Variable is good. From valve timing, valve lift, intake manifold, exhaust, ignition, fuel injection, turbocharging, cooling to lubrication, many things on today's engines can be variable. However, one thing is still fixed. That is compression ratio.

The idea of variable compression has always been fascinating. When I was still a teenager I dreamed of a variable compression engine utilizing variable-length connecting rods. That is infeasible, of course. Why do we want variable compression? Because when you turbocharge an engine you need to lower the compression ratio to avoid overheating and overstressing the cylinder head, otherwise it may cause knocking or even damages. When the engine runs off-boost, you get a weak output due to that lower compression. Suppose if we can vary the compression, using a higher ratio before the turbo gets into operation and a lower ratio under boost, we will get a perfect turbocharged engine.

In 2000, Saab announced a variable compression concept dubbed SVC (Saab Variable Compression). It implemented VC by an innovative and interesting approach - slidable cylinder head and cylinder unit. Let’s see these pictures first:


Left: high compression ratio;   Right: low compression ratio

As seen, the SVC engine have a cylinder head with integrated cylinders - which is known as monohead. The monohead is pivoted at the crankcase and its slope can be adjusted slightly (by up to 4 degrees) in relation to the engine block, pistons, crankcase etc. by means of a hydraulic actuator, therefore the volume of the combustion chambers can be varied slightly. When the piston is at top dead center, a small change of volume can lead to big change of compression ratio, ranging from 8:1 to 14:1.

SVC was cleverer than any previous attempts for variable compression as it involved no additional moving parts at the critical combustion chambers or any reciprocating components, so it was relatively simple, durable and free of leakage. The monohead was self-contained, that means it had its own cooling system, with coolant passages across the head and the cylinder wall. There was a rubber sealing between the monohead and engine block.

The VC allowed the Saab engine to run on an unusually high boost pressure, i.e. 1.8 bar (above atmospheric pressure), or about twice the boost pressure of 9-3 Viggen. It was so high that the turbochargers of its days could not provide. Therefore it employed a supercharger instead. The VC was adjustable continuously according to needs - depending on rev, load, temperature, fuel used etc., all determined by the engine management system. Therefore power and fuel consumption (hence emission) could be optimized at any conditions.

The SVC demonstrated in 2000 was a 1.6-liter 5-cylinder with 4-valve head. Max output was claimed to be 225 hp and 224 lbft, while fuel consumption was 30% lower than comparable conventional engines. Moreover, the variable compression allowed the engine to drink different Octane fuels easily, so it could be sold worldwide without needing specific tuning.

Unfortunately, the SVC never saw the light of production, probably due to its complexity and reliability issue.

Variable Compression - Nissan VC-T

Unexpectedly, Nissan becomes the first manufacturer to put variable compression ratio engines into production. The so-called VC-T (Variable Compression - Turbo) technology is to be applied to a 2.0-liter four-cylinder turbo and slated to reach the market from early 2018. The first application will be on Infiniti QX50 SUV.

The mechanism Nissan used is very different from Saab's. It varies the length of stroke by using a complicated "multi-link" system in place of conventional connecting rods. As pictured above, the multi-link system consists of an upper link, a diamond-shape center link and a lower link. The diamond-shape center link is mounted on the crankshaft journal, but it is free to swivel about the crankshaft journal. When it swivels clockwise for a few degrees, it will push up the piston thus increase the compression ratio. Vice versa, it will decrease the compression ratio when it swivels anti-clockwise for a few degrees.

The angle of the diamond-shape center link is controlled by the lower link. An electric motor (called "harmonic drive") rotates the control shaft through an actuator arm. The control shaft connects to the lower link of each cylinder through an eccentric cam. Thanks to the latter, when the control shaft rotates, the lower link can move up or down, adjusting the angle of the diamond-shape center link hence the compression ratio. If you still don't understand, you may watch Nissan's video HERE.

The VC-T can vary compression ratio between 8.0:1 and 14.0:1. The lower compression ratio is used at high-power mode because it can work with higher turbo boost pressure without causing knock. On the contrary, 14.0:1 compression is used when highest fuel efficiency is demanded. Note that this is only the theoretical compression ratio. In fact, for highest efficiency the engine uses VVT to delay the intake valve closure and implement Miller cycle combustion. In other words, the variable compression feature enables a higher, 14:1 expansion ratio to maximize the benefits of Miller cycle combustion.

Another benefit of the VC-T technology is that its multi-link geometry keeps the con-rods (upper links) more upright throughout the combustion cycle compared with conventional con-rods. This means less side force is generated, resulting in less vibration and less friction between the pistons and cylinder walls. The former saves the need for balancer shafts, while the latter enhances efficiency further.

On the downside, the multi-link system is quite cumbersome, adding considerable weight, inertia and friction. Whether these drawbacks can be offset by the aforementioned benefits is yet to be seen. However, it is certain to be more costly to build, thanks to the additional parts, especially bearings. Moreover, the mechanism is not compatible with V-engines, so its applications will be limited to high-end 4-cylinder engines that can swallow the additional costs. No wonder Nissan wants the 2.0-liter VC-T engine to replace its long-serving 3.5-liter naturally aspirated V6. Rated at 268 hp and 288 lbft of torque, its output is sufficient to do so, while fuel consumption is estimated to be 27% lower than the V6. Nevertheless, compared with some of the best 2-liter turbo engines currently on the market, it has yet to show any advantages.

Atkinson Cycle Engine

Conventional Otto cycle engines have 4 stages in each combustion cycle – intake, compression, expansion (explosion) and exhaust, each of them takes equal time and piston displacement. Atkinson cycle engines are different. They employ slightly shorter intake stroke than expansion stroke. In other words, the compression ratio is actually smaller than expansion ratio. What is the benefit of this arrangement? The answer is higher fuel efficiency. If we analyse the pressure-volume curve of combustion cycle, you will see:

For an Otto cycle engine (yellow loop), the piston starts compressing air from bottom dead center (point 1), chamber volume reduces and pressure increases until it reaches the top dead center (point 2). Then ignition takes place and the fuel-air mixture explodes, pressure surges and peaks immediately (point 3). This pushes the piston downwards, expanding the volume and decreasing pressure until the piston reaches bottom dead center (point 4). At this moment, the exhaust valves open. As the pressure of hot exhaust gas is higher than the outside world, it rushes out to the exhaust manifold quickly thus the pressure drops suddenly to atmospheric pressure (point 1). The exhaust and intake strokes are not shown here as they do not contribute to power generation. The work done (energy) produced by the combustion is the yellow area.

Now for an Atkinson cycle engine, the expansion phase is allowed to run further (the orange part), preferably until the gas pressure drops to atmospheric pressure (point 4A). This means the thermal and kinetic energy normally lost in Otto cycle through exhaust can be utilized by Atkinson cycle to produce power. This additional energy is the orange area.

Atkinson cycle engine is not a new idea. In fact, it was invented by British engineer James Atkinson in 1882. His original design was very complicated, using not only crankshaft but also an auxiliary shaft and additional linkages to allow the pistons travel shorter distance in compression stroke than expansion stroke. It was quite brilliant, but blame to this complexity and resultant extra size and weight, it was never made into commercial use on cars.

Application on hybrid cars

More recently, car makers resurrected the Atkinson concept in a bid to achieve superior fuel economy. In 1997, Toyota introduced a 1.5-liter Atkinson engine on its first Prius.
Since then all of the company's hybrid cars also adopt this kind of engines. They implement Atkinson cycle by delaying the closure of intake valves such that some fresh air is pumped back to the intake manifold in the early phase of compression stroke. This reduces the effective displacement and compression ratio. While the quoted compression ratio (i.e. geometric compression ratio) remains at slightly more than 10:1, the effective compression ratio is closer to 8:1. Except valve timing, modern Atkinson engines are exactly the same as Otto engines, thus adds no extra cost and weight.

On the downside, Atkinson-cycle engines are less powerful than their Otto-cycle counterparts of the same size and weight. This is due to several reasons: 1) Smaller effective capacity means less air and fuel involve the combustion thus less power is generated; 2) Lower compression ratio leads to less power; 3) Lower exhaust gas pressure means the exhaust gas escapes slower thus is not benefitial to scavenging effect and revvability. 4) Longer expansion stroke works against high rev. However, the lack of power is less significant on hybrid cars as fuel efficiency is placed at first priority. Also, hybrid cars can compensate the loss of power with electric motors.

Otto-Atkinson cycle engines

Thanks to stringent requirements for fuel economy, in recent years car makers also started using Atkinson principles on conventional cars. However, they use variable valve timing to adjust the closure timing of intake valves, so that the engines can run Atkinson cycle at idle and light load or switch to Otto cycle when more power is demanded, satisfying the best of both worlds. In 2012, Mazda was the first to put such Otto-Atkinson cycle engines into production. Its innovative 2.0-liter Skyactiv-G employed an unusually high compression ratio of 14.0:1 (or 13:1 for US market due to 87 Octane fuel). As a result, the effective compression ratio at Atkinson mode remained relatively high, minimizing the loss of power. In 2014, Toyota followed the same Otto-Atkinson concept with its 2.0-liter VVT-iW (on Camry), 2.0 Turbo (on Lexus NX200t) and 5.0 V8 (Lexus RC F), although their compression ratios were not as high as Mazda's. The long-abandoned invention of James Atkinson finally takes off, albeit in a much different form.

Miller Cycle Engine

Mazda's 2.3 litres Miller Cycle engine was the only one of its kind. Although it achieved 10-15 % fuel consumption reduction over comparable coventional engines, high production cost prevented it from being popular.

Invented by American Ralph Miller in the 1940s, Miller cycle was a variant of Atkinson cycle. As explained above, Atkinson cycle engines use longer expansion stroke than compression stroke to capture the residual energy that would be otherwise lost in exhaust. Therefore it returns higher fuel efficiency than conventional Otto cycle engines. However, a big drawback is lack of power compared with Otto cycle engines of the same size and construction. To address this problem, Miller cycle adds a supercharger to boost the air pressure so to restore 100% effective capacity. Of course, in order to return higher fuel efficiency, the supercharger needs to be efficient, wasting less energy than the saving gained by Miller cycle.

Valve timing of Mazda's Miller Cycle V6. Its inlet valves close at 47 degrees after BDC (bottom dead center, i.e. the lowest position of piston during a cycle). This equals to 20% of the height of stroke. In other words, during the first 20% of the compression stroke, the intake valves remain open, thus air flows out without compression. The actual compression takes place at the remaining 80% stroke, thus the effective engine capacity is only 80% of the geometric capacity. As a result, compression ratio is decreased from 10:1 to slightly under 8:1.

Mazda introduced a 2.3-liter Miller cycle V6 to its Millenia / Eunos 800 in 1994. It was claimed to consume 13% less fuel than Mazda's 3-liter conventional V6, while generated more power and a better torque curve. Nevertheless, since then neither Mazda nor other car makers followed its footprints. Why? Think about it: although it was claimed to be a 2.3-litre engine, it was actually constructed like a 3-litre engine, no matter in size, construction and materials. Then the supercharger and intercoolers added extra cost and weight. Considering the slim advantage in fuel economy, a smaller super or turbocharged engine could easily better it.

High Compression Engine - Mazda Skyactiv-G

Higher compression ratio brings higher combustion efficiency hence power. That's why automotive engineers want to raise compression as high as possible. However, a compression too high will lead to early explosion of fuel-air mixture, or what we call "knocking". Knocking is bad to engines, not only because it causes NVH but also it reduces output. When I started reading about cars, most engines in the world ran at lower than 10:1 compression. As engine management and valve-timing technology improves, nowadays the figure can be higher than 11:1. Direct injection engine may even lift that figure to 12:1 or so thanks to its cooling effect, but anything higher than that remains a dream. However, Mazda made a breakthrough with its Skyactiv-G engine in 2010. It works at an incredible 14:1 compression !

How can Mazda avoid knocking?  A crucial factor causing knocking is the high temperature of combustion chambers. Temperature in the chamber rises during compression stroke. It peaks when the piston reaches the top dead center (TDC, i.e. the highest position). At this point, knocking is most likely to occur. Obviously, if we want to reduce the risk of knocking, we had better to lower the combustion chamber temperature.

Then why is the combustion chamber so hot ? One of the reasons is the existence of residual exhaust gas, i.e. the exhaust gas that flows back into the combustion chamber during the intake stroke just before the exhaust valves close. No one can completely get rid of residual exhaust gas, because for high breathing efficiency engines always need to run with a certain level of valve overlapping (overlapping between the opening period of intake and exhaust valves). Suppose exhaust gas is 750ºC and the fresh intake air is 25ºC, and their mixture ratio is 1 to 10, you can see the residual exhaust gas can raise the combustion chamber temperature a lot. The more the amount of residual exhaust gas, the higher the combustion chamber temperature is. In other words, if we want to reduce temperature, we can reduce the amount of residual exhaust gas in the combustion chamber.

This graph shows that a 14:1 compression engine always has higher comnbustion chamber temperature than a 10:1 engine on a given residual exhaust gas level. However, if the amount of residual exhaust gas is reduced to 4 percent, combustion chamber temperature will be about the same as a 10:1 engine running with 8 percent of residual exhaust gas.

Now the question is: how to lower the percentage of residual exhaust gas?

Surprisingly, Mazda uses a very conventional approach to do that: a long, 4-to-2-to-1 exhaust manifold. On a typical inline-4 cylinder engine with short, 4-to-1 exhaust manifolds (the first picture), once the exhaust valve of Cylinder 3 opens, its exhaust pressure waves (grey area) flows through the short manifolds to the exhaust valve of Cylinder 1, which is at the end of its exhaust phase. This pumps some exhaust gas back into Cylinder 1 and becomes residual exhaust gas. When the engine is running at low speed (2000 rpm in the first picture), the exhaust pressure wave arrives Cylinder 1 early enough to cause high percentage of residual exhaust gas. As engine rev rises, the opening and closing of valves speeds up as well, thus the exhaust pressure waves of Cylinder 3 reaches Cylinder 1 at later stage, causing lower percentage of residual exhaust gas. In short, from low to mid-range engine speed the level of residual exhaust gas is pretty high for this engine configuration.

In the case of Skyactiv-G's 4-2-1 exhaust manifolds (the second picture above), exhaust pressure waves from Cylinder 3 has to travel a long way to reach Cylinder 1, by the time Cylinder 1 has already, or nearly completed its exhaust phase. Therefore the level of residual exhaust gas is much lower than the previous case, especially for low to mid-range rpm. As a result, the Skyactiv-G engine attains lower temperature in its combustion chambers and allows a higher compression ratio to be used.

Well, if the principle is so simple, why not others discovered already? It's not that simple, of course. One critical drawback of the long 4-2-1 exhaust manifold is that it takes relatively long time to heat up the NOx catalyst during cold start. In fact, this is exactly the reason why most modern production engines have abandoned this exhaust configuration - with the exception of high-performance engines which may use thin-wall fabricated stainless steel exhaust manifolds to compensate for its extra length. On cost-conscious mass production engines, cheap cast-iron exhaust manifolds are still the norm. Its extra mass and surface area absorb a great deal of heat and delay the proper functioning of catalyst. This causes difficulty to comply with emission regulations.

Mazda overcomes the cold-start problem by retarding ignition. This leads to a higher exhaust gas temperature to compensate for the long manifolds. The late ignition may result in unstable combustion. This is dealt with a specially shaped piston which concentrates the stratified air-fuel mixture around the spark plug. Other supporting features like high-pressure direct injection and six-hole injectors also contribute to the optimized combustion.

Low Compression Diesel Engine - Mazda Skyactiv-D

Unlike petrol engines, diesel engines do not have ignition system as the combustion of diesel-air mixture happens automatically under the high pressure and temperature attained during compression stroke. However, this also means diesel engines need to run higher compression ratios. It goes without saying a higher compression ratio necessitates stronger cylinder block and head, pistons, con-rods and bearings to withstand the high pressure. This explains why diesel engines are much heavier and lower revving than petrol ones. As a result, reducing compression ratio becomes the trend of diesel engine development. A decade or so ago, a typical turbo diesel engine ran about 18:1 compression. Now most employ between 16:1 and 16.5:1, while some more advanced engines are even down to 15.5:1. However, none of them are comparable to Mazda's Skyactiv-D engines, which achieve an incredibly low 14.0:1. Yes, the same as the Skyactiv-G petrol engines.

One thing preventing the use of such a low compression ratio is cold start. When the engine is cold, especially at freezing temperature, diesel engines are difficult to start. Traditionally this can be addressed by the use of glow plugs, which heat up part of the combustion chamber where fuel is injected. The lower compression ratio of Skyactiv-D just makes the matter worse as it produces lower temperature in the combustion chambers. It needs quicker acting ceramic glow plugs to deal with cold start. The use of multi-hole piezo fuel injectors, which enables more precise control of fuel spray timing and patterns, is also said to work better at cold starting.

Once the engine is started, there is still possibility of misfire. Mazda avoids this by introducing variable valve lift at the exhaust side. During the intake stroke, the VVL opens the exhaust valves slightly, drawing the hot exhaust gas back to the combustion chamber so to increase temperature. Once the desired operating temperature is reached, the VVL can be reverted to normal stage. (Note: now we understand why the high-power versions of Volkswagen
EA288 2.0 TDI engine employs variable exhaust cam phasing).

The exhaust VVL is a switchable roller rocker

Low compression produces less pollutants

Apart from weight saving, another benefit is emission. In a high-compression diesel engine, fuel is ignited as soon as it is injected into the combustion chamber due to the high pressure and temperature. This gives little time for the fuel to spread throughout the chamber. As the fuel is not sufficiently mixed with air, some fuel has no oxygen to react with, and vice versa. Consequently, the incomplete combustion generates a lot of soot (carbon particles) and NOx. Now with lower compression, the Skyactiv-D allows sufficient time for fuel to mix with air before combustion happens. It is therefore inherently cleaner, saving the need of expensive particle filter and NOx aftertreatment.

Moreover, under the current stringent emission standards, high-compression diesel engines may need to retard the timing of injection to reduce pollutants (this means the injection takes place after the piston has reached top dead center and started descending, so both pressure and temperature are reduced). This effectively shortens the expansion stroke thus waste energy (you can see it as the opposite of Atkinson-cycle engine). With lower compression, Skyactiv-D allows the fuel to be injected before piston reaches TDC, maintaining the expansion stroke thus capturing more energy.

Overall, Mazda said its 2.2-liter twin-turbo Skyactiv-D engine saves 20 percent fuel compared with its same-capacity predecessor with 16.3:1 compression ratio. The lower stress means it can switch from cast-iron block to an aluminum block to save 25 kg. Thinner cylinder head saves another 3 kg. The pistons and crankshaft are 25 percent lighter. Moreover, smaller diameter main journals can be used to reduce friction. The lower friction and lighter reciprocating mass, in addition to the use of 2-stage turbocharging, lift its maximum rev from 4500 rpm to 5200 rpm. As a result, the low-compression diesel engine behaves more like a petrol engine than ever.

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