RE: PH Origins: Variable compression ratio technology

RE: PH Origins: Variable compression ratio technology

Monday 13th August 2018

PH Origins: Variable compression ratio technology

Discover the history of a technology that's taken over 100 years to reach production status



Ever since the first internal combustion engine hit top dead centre, engineers have been locked in an inexorable battle with compression ratios. Too low a compression ratio and the engine's efficiency and performance would suffer, too high a compression ratio and catastrophic self-disassembly could occur.

Most would, as a result, err on the side of caution and opt for a safe compression ratio that would further allow the engine to tolerate low-quality fuels and poor operating conditions. This, however, often left a lot of untapped potential on the table. When cruising on the motorway at low load, for example, the chance of catastrophic pre-ignition or detonation would be low. Consequently, a high compression ratio would be ideal in this situation - as it would boost efficiency considerably.

The opposite was also the case in many instances. A force-fed engine, under lots of boost, would want a comparatively low compression ratio to avoid pistons departing via the exhaust. The wider the range of charge pressures - a key variable in supercharged and turbocharged engines - the greater the benefit of varying the compression ratio could be. Such a system would allow for the engine to attain maximum output and efficiency in a plethora of conditions, increasing its overall performance.

It was this quest for improved efficiency and reliability that led American engineer Charles Salisbury to seemingly be the first to invent an engine in which the compression ratio could be varied. His patent, submitted in June 1914 and granted in March 1917, described an engine which featured 'adjustment to vary the size of the combustion chamber by rocking the cylinder on a pivotal axis.'

This action, which would 'increase or diminish the distance between the cylinder head and the crankshaft,' permitted the compression ratio of the engine to be altered on the fly. It also, noted Salisbury, allowed for easy removal of the cylinders to permit inspection and repairs of the engine's internals.

The design was relatively straightforward, at least on paper. The cylinders and head formed one unit and were hinged to the crankcase. Bellows sealed one to the other, while a crank-based system would allow for the angle of the engine's top end to be changed. A convoluted chain drive would keep the valvetrain running and Salisbury even accounted for the need for flexible fuel, exhaust and cooling connections.


Salisbury, who had been designing industrial hardware, engines and components since the early 1900s, said: 'Efficiency may be obtained in the various running speeds of the engine, for the charge whether it be large or small may be given the same degree of compression by adjusting the size of the combustion chamber to suit the volume of the charge.'

No doubt the concept proved impractical; sealing issues aside, the operator would be required to manually adjust the compression ratio to suit the conditions - as well as dealing with fuel, ignition and throttle controls. Consequently, such a powerplant would have been complicated to use, and its potential gains limited, unless the operator knew exactly what they were doing.

The variable compression ratio - VCR - concept was not without its merits, though, and a useful and workable application was soon found. Test bed engines, in which the compression ratio could quickly be adjusted, were developed for the purposes of gathering substantial CR-related data at a rapid rate of knots. Reputedly, one such test engine was developed by esteemed engineer Harry Ricardo in the 1920s and used to study knock in aviation engines - the results from which were used to develop better ignition controls, cylinder heads, pistons and fuels.

Many companies soon began investigating VCR technology and numerous interesting solutions soon materialised - ranging from infinitely intricate and alarmingly complex configurations to beautifully engineered, straightforward designs.

One such elegant design was that produced by Dr Wilfred Mansfield of the British Internal Combustion Engine Research Association - BICERA. In an effort to reduce the complexity of a potential VCR engine, he designed a drop-in piston that featured a floating crown and internal chambers and passageways. Using a combination of oil pressure, hydraulic action and non-return valves, the chambers could be automatically filled or emptied to raise or lower the crown - increasing the compression ratio by reducing the clearance between the cylinder head and piston.

This design was patented in 1957 and later trialled by Continental in America in single-cylinder prototype engines. The results from the 1964 experiments were remarkable; the reports indicated that a planned 5.1-litre, two-stroke turbocharged V12, fitted with such pistons, could produce an almighty 830hp - over 160hp per litre - and 1,450lb ft.


The tests also suggested economy improvements in the region of 20-40 per cent compared to conventional static CR engines. Alas, it seems this project went no further - with complexity or perhaps ever-improving injection and forced induction systems making the system moot.

A Swedish engineer by the name of Alva Gustavsson then applied for a patent, in 1978, for a very different style of system. His concept featured bespoke cylinder heads that featured a small crankshaft, a set of connecting rods and small pistons - which ran in bores leading to pre-combustion chambers, or straight into the main combustion chamber itself.

Driving the in-head pistons at differing speeds resulted in a variation in the size of the combustion chamber, which could be used to tailor the compression ratio. This configuration, called the 'Alvar', was later tested by Volvo and documented in a research paper in 1998.

Yamaha also put forward a two-cylinder, two-stroke 'Super Diesel' concept in 1999 that featured a VCR system, which altered the size of the exhaust port to raise or lower the compression ratio. With the control valve open the exhaust port was shortened and effectively sat lower in the bore, boosting the amount of compression in that stroke.

It was Saab that made the first significant step towards introducing a production automotive VCR engine, though, by wholeheartedly adopting Salisbury's concept from 1914. On February 29th 2000, the Saab Variable Compression - SVC- engine was unveiled at the Geneva motor show. In a similar fashion to Salisbury's design, it used a 'monohead' configuration - a cylinder head with integral cylinders - so the compression ratio was varied by tilting the monohead in relation to the block.

The monohead could be tilted by up to four degrees, which altered the compression ratio from 8:1 to 14:1 - depending, of course, on the load at the time. An electronic control system regulated everything automatically, while the necessary canting action was provided by a crank. Rubber bellows, like those suggested by Salisbury, sealed the crankcase to the monohead.


The results were impressive. The 1.6-litre, five-cylinder engine was supercharged and produced a remarkable 225hp and 224lb ft, putting it on a par with many naturally aspirated 3.0-litre engines. Unlike those engines, however, the SVC was claimed to consume up to 30 per cent less fuel.

It wasn't the only SVC developed, either; the company had reportedly been working on VCR systems since the 1980s and had submitted a patent in 1990 detailing its initial Salisbury-emulating approach. Amusingly, its first unspecified 2.0-litre prototype is stated to have 'delivered higher torque and power output than was necessary.'

There was also a 1.6-litre straight-six prototype, which Saab chose to ditch due to packaging issues. All, though, required further development work to reach production status. By the time the five-cylinder engine was revealed to the public, Saab had not long been taken over by General Motors - which promptly halted VCR development.

This seemingly wasteful move is attributed to GM's need to cut costs but, furthermore, GM's latest displacement on demand system was in the works and would ultimately arrive in 2005. This hardware was far easier to adapt to a wider range of engines and permitted straightforward and notable reductions in fuel consumption.

Gomecsys, an engineering firm based in the Netherlands, was among those that then picked up the VCR baton. The company, which had been founded in 1997, produced its first functional prototype in 2000. Compared to some other efforts, the approach Gomecsys took was far more elegant - as it compounded all of the required hardware into one custom-engineered crankshaft. On each rod journal was a gear-driven eccentric - which could raise or lower the con rod, altering the compression ratio in the cylinder. The claim, at the time, was an 18 per cent gain in fuel efficiency with no performance drawbacks.

French company MCE5 also started working on VCR technology in 2000, later partnering with the PSA Group and other companies to develop prototype engines. The company had been founded by inventor Vianney Rabhi, whose variable compression set-up took a different approach to that of Saab's. Instead of altering the position of the head in relation to the engine's crank, Rabhi's 'VCRi' design originally entailed the use of movable sleeves in the cylinder. A rack-and-pinion system would engage with the base of the sleeves, which were threaded into the cylinder, and move them up and down as required - with their protrusion or withdrawal from the combustion chamber altering the space available and thus the compression ratio.

This fantastically involved and difficult to produce set-up soon gave way to a more effective, marketable system - one which admittedly still relied on a rack and pinion, albeit in a far more straightforward configuration. The crank would drive a connecting rod, which connected to the middle of a guiding arm. At one of end of this arm was a rack and pinion, which ran up and down in parallel with the bore axis, and atop the rack sat the piston.

At the other end of the guiding arm was an adjuster, which could be moved up and down to alter the pivoting action of the guide arm - which, in turn, would modify the stroke of the piston and alter the compression ratio. This granted MCE's engine a wide degree of adjustment, with the company claiming a CR of between 8:1 and 18:1. By 2006 it had a working multi-cylinder demonstration unit and development of its concept would continue onwards.

Lotus Engineering was another contender in the variable compression race, unveiling its 'Omnivore' engine at the 2009 Geneva motor show. The two-stroke, direct-injection engine again used a monoblock approach but, unlike other efforts, the compression was controlled by a puck in each combustion chamber. This puck, which effectively formed the top of the combustion chamber and contained the spark plug, could be moved up and down to facilitate a change in compression ratio.

The head of powertrain engineering at Lotus Engineering, Geraint Castleton-White, said at the time: 'The absence of poppet valves in two-stroke engines makes the incorporation of a variable compression ratio system relatively straightforward.

'Our research into these systems on four-stroke engines has led us to the conclusion that while thermodynamically it is a desirable technology to incorporate, practically it is very difficult, particularly taking into consideration production feasibility.'


Variable-length con rods are also among the other approaches taken to VCR, such as those developed by German engine specialists FEV - which also helped Saab develop the SVC - and Porsche.

Despite all of these efforts and countless others - there are too many to list here, including trials by Ford and Mercedes-Benz - variable compression technology was once again set to drop off the radar. This was in no doubt in part due to the increasing prevalence of mild hybrids, the electric assistance of which allowed for straightforward and significant improvements in efficiency. Ever-improving cylinder deactivation, fuel injection, turbocharging and emissions control systems further made complicated variable compression ratio set-ups redundant.

That said, the seemingly ever-defunct VCR concept suddenly sprung to life again on August 14th 2016, when Infiniti announced it had a production-ready engine. It was called the 'Variable Compression Turbo', or simply 'VC-Turbo', and made its first appearance at the Paris motor show on September 29th.

Infiniti, Nissan's luxury arm, stated that it had been developing the variable compression system for 20 years - and that more than 100 prototype powerplants had covered the equivalent of 1.9 million miles of road testing. This new design, which used a multi-link system, was claimed to be a 'driver-focused' technology that would match up well with the likes of Infiniti's steer-by-wire system, thanks to its improved performance and efficiency.

Like the SVC, the VC-Turbo's compression ratio could be altered from 8:1 to 14:1. The turbocharged 2.0-litre, four-cylinder Infiniti engine was more powerful, though, producing 268hp and 288lb ft - again, placing it in the realm of larger naturally aspirated V6s. When it made its debut in the Infiniti QX50 SUV in December 2017, the company claimed a now-familiar 35 per cent improvement in fuel efficiency compared to the outgoing V6.

Roland Krueger, Infiniti's president, said: 'Vehicle engineers believe that a breakthrough in internal combustion technology would come from mastering the variable compression ratio. Infiniti will be the first to bring this technology to the market in 2018. This new generation of powertrains will help our global growth and expansion of the Infiniti product portfolio.'


It might have seemingly gone quiet on the Infiniti VC-Turbo front but, in 2019, a larger 2.5-litre version will be used in the Nissan Altima. The likes of MCE5 are also still persisting with VCR technology; in 2017, it developed and tested a VCRi engine for the Chinese government-owned Dongfeng Motor Corporation - engines which are now reputedly being phased into Dongfeng's industrial line-up.

As the technology becomes more proven - and as companies continue to strive to eke every remaining ounce of efficiency out of the internal combustion engine - perhaps the oft-abandoned concept of VCR will finally have its time in the spotlight.

Author
Discussion

RumbleOfThunder

Original Poster:

3,546 posts

202 months

Monday 13th August 2018
quotequote all
It's frustrating to see such obvious and viable ways to lengthen the life of the ICE and yet the technology just sits on the shelf because OEM's are too lazy or too scared of being the ones to risk their margins and have a stab at it. GET IT INTO PRODUCTION! Move to solenoid valves and throw out camshaft arrangements too, another viable way of improving efficiency by a huge amount. koenigsegg have had a camless Saab running around for years now.

JMF894

5,475 posts

154 months

Monday 13th August 2018
quotequote all
Mentioning Saab and GM in the same sentence still makes my blood boil. Oh. Damn.

anonymous-user

53 months

Monday 13th August 2018
quotequote all
RumbleOfThunder said:
It's frustrating to see such obvious and viable ways to lengthen the life of the ICE and yet the technology just sits on the shelf because OEM's are too lazy or too scared of being the ones to risk their margins and have a stab at it.
Says the armchair expert with nothing more to risk than sounding stupid!


OE automotives are Conservative with a capital C for many very good reasons, mainly involving not bankrupting their company and putting 10's of thousands of people out onto the dole.........



VCR is a tech that actually, despite what you might read, isn't as useful as many make out. Conventional engines already have a dynamically controllable effective compression ratio (it's called a throttle) and add turbocharging into the midst, and the real world benefits from VCR become quite small. Then, add in the fact that no matter how variable you make the CR, the engine is never going to be able to fill the fuel tank back up when you lift off, and suddenly, compared to electrification (hybrid, let alone full EV) you have a complex, expensive system that gets you just a few percentage points of gain.

As ICE's get more and more electrified, and the engine becomes more of a range extender, simplification, light weighting and low cost starts to become the watch word, and VCR will quickly fall by the wayside.......



Mr2Mike

20,143 posts

254 months

Monday 13th August 2018
quotequote all
RumbleOfThunder said:
koenigsegg have had a camless Saab running around for years now.
Have they? There was (quite rightly) a lot of fuss when the Freevalve Saab was showcased, but there's been little in the way of updates for the last couple of years.

jagnet

4,095 posts

201 months

Monday 13th August 2018
quotequote all
Max_Torque said:
OE automotives are Conservative with a capital C for many very good reasons, mainly involving not bankrupting their company and putting 10's of thousands of people out onto the dole.........
All credit to GM, they've managed to achieve that whilst being conservative.

unsprung

5,467 posts

123 months

Monday 13th August 2018
quotequote all
PH article said:
A force-fed engine, under lots of boost, would want a comparatively low compression ratio to avoid pistons departing via the exhaust.
hehe

another illuminating lecture, this article (thanks!)

even if this particular technology falls by the wayside, there are others that promise significant improvement to efficiency and emissions; hooray internal combustion

on a related note...

just when you think you understand something like displacement on demand, it, too, becomes disrupted

in this case, by dynamic skip fire:
https://www.popularmechanics.com/cars/car-technolo...

"Software determines how much torque the vehicle needs and -- as often as 200 times per second on an eight-cylinder engine and 100 times per second on a four-cylinder -- it shuts down every unnecessary cylinder in order to save fuel."

"It can pick any combination of cylinders, calculating which ones pumping away together would create the least combined noise and vibration for that moment's driving conditions."











southerndriver

250 posts

73 months

Wednesday 15th August 2018
quotequote all
Interesting article, particularly the history of the subject. Infiniti’s forthcoming introduction of an engine featuring VCR could be a milestone. Since Nissan and Renault are joined at the hip these days I wondered which of them has actually done the work on this. If it’s Renault, with their appalling track record in recent years that includes Megane and Laguna II, I wouldn’t touch this with a bargepole. I would give the engine two years before it sts itself.