Not all gasoline engines need spark plugs to ignite their fuel. In a conventional internal combustion engine (ICE) cycle, a mixture of air, spark and fuel come together over the course of four strokes to create power and move a vehicle forward. Diesel engines do not require spark plugs to ignite the mixture and produce combustion. After the pistons compress only the intake air to a very high level, combustion occurs spontaneously when fuel is then injected into the combustion chamber at the appropriate time. Then there’s the third type of ICE—the homogeneous charge compression ignition (HCCI) engine.
Think of HCCI variable combustion as the best of both the gasoline and diesel worlds. HCCI engines feature the same four strokes, but there’s simultaneous combustion of air and
fuel, which is spread via a homogeneous mixture. The two are evenly pulled into the cylinder, where the mixture is ignited by way of compression, rather than a spark, similar to a diesel engine.
HCCI can generate more horsepower, gain higher fuel efficiency and give off cleaner emissions. But it also presents a difficult challenge of balancing internal temperature: Too cold, the performance of the ignition system suffers; too hot, damaging engine knock and misfires can occur. Theory aside, finding a “Goldilocks” solution has been a struggle.
The search began nearly a hundred years ago, when Sir Harry Ricardo first described the theoretical performance advantages of variable compression ratio (VCR) engines, the foundational basis for HCCI. Since then, automakers have mostly struggled to develop and market a real-world, successful “Goldilocks” solution. Early concepts include Daimler AG’s DiesOtto, General Motors’ HCCI and Hyundai’s gasoline direct injection compression ignition (GDICI). Each attempted to convert gas engines to dual-mode operation that employed the spark plug under full load and/or idle conditions and compression ignition during steady-state cruising.
However, none made it to production series automobiles. The problem with putting theory into practice is that unanticipated realities can derail intentions when scaling up production. For example, under compression-ignition operating conditions, the early developers found peak cylinder pressures were so great that the engine block and lower end (crank and bearings) needed to be reinforced, which added cost and mass. In addition, hoped-for emissions aftertreatment savings were negated by the need for extra exhaust-gas recirculation measures. Consequently, despite its promise, HCCI development by these early protagonists slowed to a stop.
Behind the scenes, however, other mainstream automakers and some boutique engine manufacturers continued the pursuit of HCCI, particularly with new technologies they developed for these variable compression engines. Nissan/Infiniti, Volkswagen, Mazda and others come to mind. Their efforts have put HCCI in the spotlight again. This time, however, variable compression engines are actually entering the marketplace in production vehicles, which will drive the opportunity to service and repair them your way.
Nissan: Truly Variable Compression
“Variable compression is poised to power future gasoline engines,” shared Shinichi Kiga, Nissan’s chief powertrain engineer for its Gasoline Engine Project Group. “Until now, in every production HCCI engine ever built, the compression ratio has been fixed as a result of the dimensions, components and engine layout. Nissan is the first automaker to transform HCCI theory into marketplace reality.”
Of note, the engine’s compression ratio automatically adjusts to the driver’s inputs to offer any compression ratio between 8:1 (typical for high-performance engines) and 14:1 (used by many manufacturers for high efficiency). To achieve this, Nissan R&D for its VC-Turbo engine integrated three major innovations:
Harmonic Drive Gear. By sensing a vehicle’s driving condition and driver inputs, this electrically powered reduction gear rotates accordingly to deliver the compression ratio required at any given moment, between a range of 8:1 and 14:1. The gear drives a multilink mechanism that moves the connecting control rods at the base of the engine to vary the piston stroke position for all four cylinders at the same time. This adjusts the height of the top dead center (TDC) of the pistons, thereby varying the compression ratio. Under hard acceleration or at heavier engine loads (e.g., when overtaking another vehicle or driving uphill), the engine shifts to a lower compression ratio. In contrast, at steady or slowing driving conditions, the engine defaults to a higher compression ratio to maximize efficiency.
Reduction of Mass and NVH. Nissan also saw an opportunity to reduce mass by amalgamating the exhaust manifold and cylinder head into one assembly. The innovation improved packaging, enabled a more intelligent integration with the turbocharger and catalytic converter and allowed a reduction in noise-vibration-harshness (NVH) by a third and avoided secondary harmonic vibrations. This eliminated the need for a balancer shaft while still delivering a smooth-running and quiet engine. Specifically, the vibration noise level in the new VC-Turbo engine is just 10 decibels, which compares favorably to the benchmark average of approximately 30 dB in the company’s larger V6.
Enhanced Fuel Management Strategy. The VC-Turbo engine’s fuel strategy employs a combination of both multipoint injection (MPI) and direct injection (DI). MPI allows for earlier mixing of air and fuel to increase engine efficiency at low loads, while DI improves combustion efficiency and performance due to the heat-absorbing effect of fuel vaporization, which aids cylinder cooling and allows the engine to avoid knocking at higher compression ratios. The engine can switch instantly between DI and MPI at regular engine speeds, without compromising the fuel injectors’ ability to work, regardless of engine speed and load. In addition, the engine can switch automatically between Atkinson and regular combustion cycles to further optimize engine performance.
“Our new VC-Turbo, a turbocharged 2.0L inline four with direct fuel injection engine, delivers a peak 268 horsepower and 288 foot-pounds of torque,” Kiga explained. “It first debuted in the 2018 Infiniti QX50 premium crossover, replacing the former 3.5L V6, and will soon become the basis of a new engine family for all Renault Nissan Alliance brands.”
Mazda Leverages Spark Ignition
Mazda solved the technical challenges HCCI-based engines have long posed with its new spark controlled compression ignition (SPCCI) gasoline engine, which will be introduced as the SkyActiv-X in the 2019 Mazda 3. The automaker uses proprietary spark plug technology that enables the new engine to run on compression only or use a spark plug to control ignition timing.
SPCCI combines more than three decades of Mazda research into a refined, torque-rich package. Conventional compression-ignition needs very specific conditions to operate efficiently—usually under very light loads—but Mazda’s breakthrough uses a traditional spark plug to begin the combustion process, then leverages the pressure rise from the resulting flame kernel to trigger compression ignition in the remainder of the cylinder.
Recently, Mazda shared some of the technology driving its new SPCCI engine:
Air Supply Technology. To enable the SPCCI engine to operate over a wide range of speeds and loads, a highly responsive air supply unit helps raise the ratio of compression ignition combustion and facilitates smoother switching between HCCI and spark ignition modes.
“The combination of SPCCI and the air assist mechanism can reach compression ratios as high as 30:1, achieve 10% to 30% torque improvement, up to a 20% increase in fuel efficiency and substantially reduced NOX emissions,” explained Dave Coleman, a Mazda development engineer.
Air Piston Compression. The innovative control system for SPCCI-based combustion extends the typical HCCI range out to cover a much larger percentage of the load map. Right after the piston starts moving from TDC, combustion takes place spontaneously and rapidly, which enhances the force to push the piston and allow combustion to continue for a longer time. Also, SPCCI employs a spherical flame front, expanded by spark ignition that acts as a second piston—like an “air piston”—to further compress the air/fuel mixture. This results in improved compression ignition, whatever the engine’s operating temperature.
Controlled Compression Ignition (CCI). “Controlled compression ignition is an original Mazda technology that enables a super lean burn at twice the ideal air/fuel ratio,” explained Kiyoshi Fujiwara, Mazda’s director and senior managing executive officer. “Even lean air/fuel mixtures will ignite and burn in many places simultaneously if highly compressed. Delivering stable gasoline combustion at all speed and load ranges requires an engine that supports both spark ignition and HCCI, which by definition requires a spark plug as part of the engine design. Mazda turned that into an advantage, and uses a spark plug as the means to control ignition timing, expand the range for compression ignition and enable a smooth switchover between modes.
“The result is SPCCI, a clean engine that offers high torque and excellent fuel economy thanks to super lean combustion, which has never been possible before with spark ignition,” Fujiwara noted. “With high efficiency across a wide range of rpms and engine loads, the new SPCCI engine kicks in when the temperature is extremely low, enables stable switching between HCCI and spark ignition, allows much more latitude in the selection of gear ratios and provides both superior fuel economy and driving performance. Look for other models in the near future to include optional SPCCI engines.”
VW: A Less Expensive HCCI Solution?
“The needs of our customers and industry will change more fundamentally in the next ten years than they have in the last hundred,” stated Matthias Miller, CEO of the Volkswagen Group (VW). “Our new Budack compression cycle engine—a 2.0L four-cylinder, turbocharged direct-injection internal combustion engine—incorporates a number of significant changes that will advance Volkswagen’s twin goals of power and efficiency and help us keep ICEs relevant and affordable over that timeframe.”
Code-named EA288 Gen3B, the new engine demonstrates VWs move toward smaller and affordable turbocharged ICEs that offer the fuel economy benefits of downsizing with the power of a larger-displacement unit. The engine debuted in the 2018 Tiguan model. Peak 184 hp kicks in at 4400 rpm, nearly 400 revs sooner than in the 2017 edition, and that output level is maintained until 6000 rpm. Maximum torque of 221 ft.-lbs. is achieved at 1600 to 3940 rpm.
Some of the key tweaks and technologies in the new VW engine include:
Budack Combustion Cycle. What makes this advanced engine unique is VW’s leverage of modifications to the conventional four-stroke Miller combustion cycle. Whereas the traditional Miller cycle closes the intake valves just before the end of the intake stroke, the Budack combustion cycle closes the intake valves earlier. Thus, as the piston goes down with the valve closed, the pressure inside the combustion chamber drops, then rises during the compression stroke. This yields a lower effective compression ratio, which makes it easier for the power stroke to turn energy from a longer effective combustion into downward motion of the piston.
Friction-Reducing Hardware. To produce better fuel economy and performance, the EA288 Gen3B also utilizes a more responsive turbocharger, a longer (92.8mm) piston stroke, lower-tension piston rings, modified piston crowns, a redesigned combustion chamber (which raises the compression ratio to 11.7:1, up from 9.6:1) and new Bosch TSI engine injectors that push fuel into the cylinder at a higher maximum pressure (250 bar or 3626 psi), with up to three injection sequences per stroke, depending on conditions. The result: an 8% reduction in engine friction with improved power, efficiency and responsiveness over the range of driving most customers use every day.
New Engine Management System. The new powerplant employs four core processors that monitor and adjust the system to reduce emissions. For example, lower brake mean effective pressure at full load in the EA288 Gen3B allowed the crankshaft main bearing diameter to be reduced from 52mm to 48mm and the balance shaft chain to be narrower.
Innovative Variable Valve Timing (VVT). In addition, the engine features a novel VVT system on the intake camshaft. Depending on engine load, it’s possible to switch between short and long valve opening. At idling speed and under partial load, the valve opening is shorter. When the engine is placed under greater load, a switch to the camshaft lobe that opens the valve for a longer period is made and the driver can make use of the full power and torque of the engine.
“The bottom line is VW’s new EA288 Gen3B engine is a lower cost HCCI alternative when compared to Nissan and Mazda,” explained inventor Ralf Budack, a VW development engineer. “Following the launch in the 2018 Tiguan, the EA288 Gen3B will be offered in other models. For example, the EA288 Gen3B could well serve as an efficient replacement for the diesel engines the automaker no longer plans to sell in North America.
As automakers approach ever-increasing fuel economy and emissions regulations with internal combustion engines, they’re always on the lookout for more affordable and effective solutions. The stakes are greater performance output per liter of displacement, lower CO2 emissions, lighter vehicles and other operating and cost efficiencies.
For example, Nautilus Engineering continues to develop its dynamic multiphasic combustion engine, a four-stroke, six-cycle design that utilizes various HCCI cycles. “Nautilus now holds patents to make controlled HCCI combustion possible for a wide range of engines,” said CEO Matthew Riley. “The engine’s defining feature is a piston with a top protrusion that creates the engine’s ‘primary’ combustion chamber as the piston approaches the top of the stroke. This HCCI engine will be cheaper to build than a conventional spark-ignition gasoline engine, requires no special manufacturing investment and will return up to 30% improved fuel economy and near-zero NOX emissions.”
Koenigsegg Automotive is nearing the completion of its “camless” engine, which leverages a new type of valve system. The goal is “a camless engine that features independent control over each valve by replacing the camshaft with pneumatic actuators and software to run it,” said CEO Christian Von Koenigsegg. “Pneumatic valve actuators open valves, which are then closed by air pressure or springs. In addition, the pneumatic valvetrain draws less energy than engines with conventional cams. Each of the valves can be controlled individually, which enables infinitely variable valve lift and duration as well as easy cylinder deactivation. Our last remaining hurdle is to reduce noise levels and integrate that into our prototype engine. We’re very close to realizing that.”
Nor are mainstream automakers and suppliers standing pat. In the past few months, GM/Honda and BMW/McLaren have announced new powertrain collaborations focused on unique combustion strategies. Delphi and Tula have made major strides in 48V systems with aggressive and effective cylinder deactivation capabilities. Even stalwart Chevrolet has embraced combustion efficiency, in the light-duty pickup truck segment, of all places.
The GM subsidiary recently announced that 2019 Silverado pickups would be available with a downsized four-cylinder 2.7L engine, the first ever offered by an automaker in the work truck category. “The new turbocharged and intercooled gasoline 2.7L DOHC I-4 engine runs on 87-RON gasoline, and is more powerful and efficient than the 4.3L V6 it replaces,” shared chief engineer Tom Sutter. “The engine’s 10:1 compression ratio delivers 310 horsepower and 348 foot-pounds (472Nm) of torque, utilizes an all-new electromagnetically actuated camshaft design and includes fuel-saving cylinder deactivation technology.”
The point is that HCCI has arrived. Different solutions are already in the marketplace, and more are on the way. Development is ongoing, and don’t be surprised to see in the near future solutions that combine HCCI with other advanced technologies. Check out the service information and procedures already available from the automakers selling HCCI-equipped vehicles today. And begin looking for training from automakers and aftermarket trainers. It’s only a matter of time before HCCI vehicles needing service and repair arrive at your shop.