Aiming to be World’s Number One Naturally-Aspirated
Engine Manufacturer
Sports Concept SSM developed into the S2000
On the 1990s arrived, the competitiveness of Honda’s 2.0L-class 4-cylinder engines began to decline, prompting plans to be made to develop a new engine for the next generation.
Preliminary development started in 1994. The need to unify the multiple lineups of engines made it necessary to create a versatile framework, which caused many headaches. In particular, compactness was essential to enable installation in compact cars, and a larger bore pitch of 94 mm was selected to cover engines with displacements up to 2.4L. With the Civic’s 1.7L engine, bore pitch was 84 mm, and a simple comparison of the overall length of the engine block showed that it was 10 mm longer per cylinder, or 40 mm longer for four cylinders. To mount the engine in a Civic, it would need to be made significantly more compact. In addition, the camshaft drivetrain had to be common so that DOHC or SOHC could be switched depending on the model by simply replacing the cylinder heads on the top of the engine. In addition, in order to enable the installation of a secondary balancer shaft that realizes excellent quietness, a structure that can be detached as a unit was also considered. Moreover, the layout of the exhaust system was changed from the conventional forward exhaust to rear exhaust in order to shorten the distance between the catalyzer and exhaust system to comply with stricter exhaust emission regulations. This preliminary development technology led to the birth of the K engine, which was later adopted in a wide variety of vehicles as the core engine, and the F20C*12, the ultimate naturally aspirated VTEC engine. However, the first prototype engine was called “excellent, but no flair,” at its evaluation meeting.
At the same time, the model exhibited at the Tokyo Motor Show as the Sports Concept SSM reference model received high acclaim, and consideration began to be given to mass production. The concept model at the time had a 5-cylinder engine, but it was decided to switch to 4-cylinders, using the engine currently under development. From this point on, the mass production development of the Sports Concept SSM, the F20C that would later be installed in the S2000, and the K engine would proceed separately.
- S2000 engine
Developing the World’s Most Powerful*13 125 PS per Liter
F20C Engine
Technology to improve intake/exhaust ratio and high compression ratio
Friction reduction technology, roller coaxial VTEC rocker arms
Yoshiaki Akimoto, who had been involved in the development of the F20C from preliminary research, became the project leader (PL), and began development of a new inline 4-cylinder engine with “flair” based on the previous development. The goal was to create a lightweight, compact engine with high revs and output, advanced environmental technologies, and high response. The development team set a target of 240 PS. At the same time, the U.S. Low Emission Vehicle (LEV) regulations were introduced, and it was decided that all high output engines would also be LEV-compliant. LEVs were subject to strict standards that meant almost no emissions, and the reality was that 200 PS was the limit for meeting these standards. The development team re-set the target to 220 PS. By building on technologies such as improved intake and exhaust efficiency, higher compression ratios, and reduced friction, the team was able to achieve 225 PS, exceeding the target.
In parallel, development of the model was underway, and an evaluation meeting was held once the specifics of an open sports car, the S2000, were determined. At the meeting, when Kawamoto, then the fourth president, asked about the development status of the engine, the team responded that they had achieved 225 PS. Hearing this, Kawamoto set an insurmountable hurdle, saying, “Make an engine that revs up to 10,000 rpm smoothly.”
Akimoto discussed the matter with his boss, Yutaka Otobe. When Otobe asked Akemoto, “How do you propose to go up to 10,000 revs? Akimoto replied, “If it was a 1.7-liter engine, we could make it go up to 10,000.” However, the 1.7L engine was not enough for the S2000, which was expected to weigh in at the 1,200 kg range.
Kawamoto agreed, saying, “We'll have to convince him [Kawamoto] that we can get an even 250 PS out of it by going up to 9,000 revs,” and he managed to get Kawamoto’s approval. This gave birth to the story of Kawamoto's “make it an even 250 PS.” This was because Otobe had only talked about horsepower externally.
- From 1999 to 2010 as naturally-aspirated engine. Honda research.
Pulverized Pistons, Prototype Engines that Keep On Breaking
Forged aluminum pistons Carburized and forged con-rods
High strength single valve spring / valve
A 2.0L engine delivering 250 PS is 125 PS per liter. At the time, this was in the realm of high-performance racing engines. Even Akimoto, who was responsible for the engine thought it would probably not produce 250 PS. However, the development team was enthusiastic with the goal of creating the world’s best engine, and had no intention of giving up. Everyone wanted to see the pinnacle that no-one had seen before.
They managed to overcome difficulties and realize the engine,but the hardships involved were beyond description. With a fixed bore pitch of 94 mm, the stroke was inevitably long, and with an average piston speed of 25.2 m/s at 9,000 rpm, the engine entered an unknown territory far beyond the F1 engines of the time, breaking down every time it was on the test bench. Unthinkable damage occurred, such as valves knocking the pistons, and the engine had to contend with intense inertial forces. After trying different types of pistons and connecting rods but with no improvement, the pistons were changed from cast aluminum to forged in order to reduce weight and increase strength. Forging has toughness, and when it breaks, remains in place instead of shattering, allowing the team to assess the cause of the breakdowns. As a result, they discovered for the first time that the dynamic valve system broke at the same time as the pistons, and measures were taken to somehow fix the problem.
Once the engine actually worked, the next step was to solve smaller issues. Digital tools were not yet invented so, for example, sensors capable of measuring piston behavior at the micron level were placed inside the engine, and analog data from the sensors was imported into a personal computer. As a result of creating analysis software to visualize piston behavior, the team discovered that the piston moves in the direction opposite to conventional wisdom when the engine speed exceeded 8,800 rpm. This led to the idea of reversing the offset of the piston width and pins. For environmental performance, the team decided to adopt a metal catalyzer and secondary exhaust air system, which had been promoted in a preliminary research theme, to raise the temperature of the catalyzer at an early stage to prevent emissions.
S2000 powered by F20C engine
The young developers' efforts to develop the world’s best engine with 125 PS per liter bore fruit in the form of the F20C, a 2.0-liter inline 4-cylinder naturally-aspirated DOHC VTEC engine with a redline speed of 9,000 rpm and a maximum output of 250 PS. This engine powered the S2000 launched in April 1999. The 125 PS per liter record for a mass-produced automobile engine was not broken for 10 years, until 2010. And this engine, the pinnacle of Honda’s naturally aspirated VTEC engines, is still the benchmark for sports engines in 2023. The S2000, which was discontinued in 2009, sold 110,000 units in Japan, the United States, and Europe. There is no doubt that the S2000’s appeal as a true open-top sports car was driven by its high-performance engine, the pinnacle of VTEC, which remains unfaded over the years.
F20C engine. Held the record of the only mass-produced automobile engine to achieve 125 PS per liter, for ten years until 2010.
Developing New Frame Engine to Expand K Engine
to Wide Range of Models and Uses
The main theme of development of a new 2.0L-class inline 4-cylinder K engine, Honda’s new core engine, was to dramatically improve environmental performance without reducing output. To achieve this, it was necessary to improve combustion efficiency, and Honda had decided to combine Honda’s outstanding VTEC, with a continuously variable valve timing control mechanism, VTC (Variable Timing Control), in order to improve efficiency throughout the entire engine speed range.
In contrast to the prestige of developing the new inline 4-cylinder F20C, Tomonori Niizato was selected as the sub-project leader (at the time) to design the mass-production K engine, which could be considered the mainstay of Honda’s engines.
Since the engine block length could not be reduced due to the fixed bore pitch, Niizato focused on the belts that drive the auxiliary parts. For simplification, he considered making the secondary balancer and oil pump more compact and placing them in the oil pan. As specifications of the secondary balancer needed to change depending on the type of car, this problem was simultaneously solved. Secondary balancers conventionally had a balancer shaft through the cylinder block and were belt-driven, but this belt took up a lot of space, and on the other hand, removing the balancer created a large cavity in the cylinder block. The solution was to make a cassette-type module and place it in the oil pan at the bottom of the block, eliminating the belt and allowing specification changes.
i-VTEC system
By positioning the oil pump close by, and by driving the secondary balancer and oil pump with a single drive shaft, an extremely lightweight and compact structure became possible. This was a revolutionary idea, but in realizing it, Niizato went through an ordeal that he had never experienced before in his life as an engine designer.
The secondary balancer rotates twice as fast as the engine, meaning the linked oil pump also rotates that fast. When the engine rotates at 7,500 rpm, the oil pump would rotate at 15,000 rpm. This speed was unheard of. After careful calculation and design, the time finally came to test the prototype engine. The oil pump broke down as soon as the first prototype engine was fired up. When the engine was opened up, the oil pump had scattered in pieces in the oil pan.
When the oil pump breaks, the engine seizures, ruining the entire engine. Without a prototype engine, the entire development process would be stalled. This was a major problem. The team hurriedly planned corrective measures and flew to Gunma, where the company commissioned to prototype and build the engine was, and begged them to implement the fixes. The prototype engine, however, broke down again, and two major design changes followed. The delay in the development schedule would lead to an enormous increase in development costs. Niizato was humbled: At first scorned, when someone was in real trouble, Honda’s culture is that the surrounding people would kindly help out, as was Niizato’s case. After this fiasco, the modular oil pump, with flexible specifications, became a reality. All subsequent Honda engines adopted this system. Honda’s new standard technology was born from Niizato’s pain and his refusal to give up.
Honda’s Excellent Core Engine
The K engine, for the purpose of downsizing and maintenance-free operation, used a slim silent chain for the cam drive system, instead of a twice-as-wide rubber belt that required regular replacement. In addition, the two conventional auxiliary belts were replaced with a serpentine belt system*14 in which all auxiliary components were mounted on an integrated bracket larger than the cylinder block and rotated by a single belt, resulting in significant downsizing. Previously, two belts were required, as the hydraulic pump for power steering, an important safety component, had to rotate independently, but with the evolution of belt durability and the proliferation of electric power steering, the system could be simplified, and this specification later became the standard for Honda engines.
In addition, the inside of the integrated bracket was hollowed out to create a water circuit that supplies water from the water pump to the cylinder block. This water circuit also served as a breather chamber, and while sealing the water and gases, it was an excellent dual-purpose solution that could support the heavy auxiliary components and withstand vibration. To increase engine rigidity, the cylinder block was divided at the crank center and a ladder frame lower block system was adopted, with five crank journals as integral parts to achieve high rigidity.
As mentioned, the K engine was initially considered for not only the current DOHC engine but also SOHC, but since it was predicted that SOHC would make it difficult to adopt environmentally friendly technologies such as VTC in the future, development was discontinued and the engine was made exclusively for DOHC, but the cylinder head size was as compact as an SOHC.
On the other hand, a rear exhaust layout was adopted to shorten the distance between the exhaust and catalyzer to improve environmental performance, but this required many variations of exhaust systems to accommodate different types of vehicles, which made it difficult for exhaust system designers.
Rear-facing exhaust system
The 2.0L / 2.4L DOHC i-VTEC K engines, Honda’s core engines
In October 2000, the K-type engine was introduced as the new generation DOHC i-VTEC engine. With a single basic structure, it powered an extremely wide range of models, customized for each, and became the Honda’s undisputed core engine, recording an annual production of 900,000 units in some years. With various technologies such as lean-burn and direct injection, it reestablished the performance advantage of Honda engines. Niizato, who was involved in the design, said, “It was not a spectacular technology, but we were able to create the framework of a new Honda engine on my initiative. Even if it is a modest technology, when you decide to do it yourself and work through it, technical development is a lot of fun. No matter what challenges we encounter, we can usually handle them.”
- A single belt that drives multiple auxiliary units. Serpentine as in “snake-like.”
