POWER UNIT
The CBR250R engine was developed by aiming at a global single-cylinder engine that transcends regions, while being sporty and eco-friendly at the same time, with a look ahead at the next generation.
What should a next-generation liquid-cooled 250cc single-cylinder engine that is also ecofriendly be like?
What should the output characteristics of a 250cc single-cylinder sport bike be when the comfort and the ease of handling that can satisfy customers all over the world are taken into account?
How can an engine that is light and compact and yet has a high-quality feel be able to support the ride as the rider wants it?
With these questions in mind and aiming at the output characteristics that would realize a light and sporty maneuverability in the complete vehicle while retaining the advantages of fuel economy and ease of handling of conventional single-cylinder engines, we have developed a new 250cc single-cylinder engine that is light and compact and runs smoothly not just at low rpm but also in the high rpm range.
ENVIRONMENTAL PERFORMANCE
To fulfill basic performance requirements as a sport bike while setting higher targets for environmental performance, a DOHC was chosen as the valve system. DOHC improves combustion efficiency by reducing the weight of the reciprocating portion of the valves. This selection also allows us to freely choose the included valve angle, the port shape, and the shape of the combustion chamber. The choice of DOHC contributes to improved product appeal as a sports bike as well as to performance.
For the valve system, a roller rocker arm was adopted by a DOHC engine motorcycle for the first time in the world. A low-friction valve train with a smaller cylinder head was achieved through an ultra-compact layout for the roller rocker arm.
The choice of a shim design for valve tappet adjustment reduced the rocker arm weight, while friction was reduced by setting the valve spring load to a low level.
For better maintainability, the shim can be replaced without removing the camshaft.
To reduce blow-by gas and oil consumption, a spiny sleeve was adopted for the cylinder sleeve. Small spines have been added to the outer surface to improve cooling performance and help reduce distortion of the inner cylinder's shape. In addition, centrifugal casting allowed a thin, uniform wall thickness, which aids weight reduction. For emission measures, an O² sensor is combined with the built-in air induction (AI) system and a catalyzer is fitted inside the exhaust pipe to comply with Euro 3 emission regulation. The power unit complies with other environmental regulations such as Thailand's sixth emission standards, for which the evaporator system (evaporative emission control system) was put in place for the Thai model.
ENGINE PERFORMANCE:
In the area of engine performance, a short-stroke engine (bore stroke: Φ76 × 55) was chosen to improve responsiveness in making the bike's sporty characteristics easy to handle. The resulting bore stroke is on a par with that of CBR1000RR, Honda's high performance, super sport bike. In designing the crankshaft, no efforts were spared to reduce weight in order to lower the inertial mass while achieving additional weight reduction in the piston and connecting rod. Sufficient cooling performance was ensured by conducting a high-efficiency cooling water flow analysis based on a CAE simulation to give higher output. To ensure that output characteristics are stress-free and powerful from low to high rpm ranges, charging efficiency was improved. Additional measures for higher efficiency include straightening from the air cleaner to the exhaust pipe. Valve stems have been made thinner, although valve diameters are large for both intake and exhaust. This valve design is coupled with a wide opening angle and a high-lift cam to improve intake/exhaust efficiency. To achieve output characteristics that are easy to handle at low speed and smooth rev-up at high rpm, the intake/exhaust systems were thoroughly analyzed to adopt the ideal port/pipe length and bore size.
QUIETNESS:
Honda's original new-generation crankshaft mechanism was used to realize an engine full of high-quality feel while maintaining high output.
First, a metal bearing (half-split, press-fit) was chosen for the crank journal for the first time on a Honda single-cylinder motorcycle. A cast-iron bush was selected for the crank bearing section in order to improve the rigidity of the crankcase housing and control changes crank journal's oil clearance arising from thermal expansion. In this way, the design takes productivity into account with an eye toward a global roll-out, while improving quietness at the same time.
Second, while using a built-up type crankshaft that allows the big end of the connecting rod to be used as a low-friction roller bearing, the optimal crank web shape was realized as a result of a computer analysis of strength and rigidity. This ensures high rigidity on a par with a solid type crankshaft.
Crankshaft rigidity is enhanced further and quietness is improved by placing the balancer's driving gear on the right cover inside the clutch housing -a design that narrows the distance between the left/right crank bearings and places a ball bearing at the tip of the crankshaft right side. For vibration, a primary balancer was chosen to produce a high-quality single-cylinder engine.
The CBR250R engine was developed by aiming at a global single-cylinder engine that transcends regions, while being sporty and eco-friendly at the same time, with a look ahead at the next generation.What should a next-generation liquid-cooled 250cc single-cylinder engine that is also ecofriendly be like?
What should the output characteristics of a 250cc single-cylinder sport bike be when the comfort and the ease of handling that can satisfy customers all over the world are taken into account?
How can an engine that is light and compact and yet has a high-quality feel be able to support the ride as the rider wants it?
With these questions in mind and aiming at the output characteristics that would realize a light and sporty maneuverability in the complete vehicle while retaining the advantages of fuel economy and ease of handling of conventional single-cylinder engines, we have developed a new 250cc single-cylinder engine that is light and compact and runs smoothly not just at low rpm but also in the high rpm range.
ENVIRONMENTAL PERFORMANCE
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| Silent cam chain (SV chain) |
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| Roller rocker arm |
For the valve system, a roller rocker arm was adopted by a DOHC engine motorcycle for the first time in the world. A low-friction valve train with a smaller cylinder head was achieved through an ultra-compact layout for the roller rocker arm.
The choice of a shim design for valve tappet adjustment reduced the rocker arm weight, while friction was reduced by setting the valve spring load to a low level.
For better maintainability, the shim can be replaced without removing the camshaft.
 |
| Cross-sectional view of cylinder head |
To reduce blow-by gas and oil consumption, a spiny sleeve was adopted for the cylinder sleeve. Small spines have been added to the outer surface to improve cooling performance and help reduce distortion of the inner cylinder's shape. In addition, centrifugal casting allowed a thin, uniform wall thickness, which aids weight reduction. For emission measures, an O² sensor is combined with the built-in air induction (AI) system and a catalyzer is fitted inside the exhaust pipe to comply with Euro 3 emission regulation. The power unit complies with other environmental regulations such as Thailand's sixth emission standards, for which the evaporator system (evaporative emission control system) was put in place for the Thai model.
Cylinder sleeve
AI/O² sensor
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| Catalyzer |
AI pathway
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| Cross-sectional view of straight port's form |
QUIETNESS:
 |
| Primary balancer |
First, a metal bearing (half-split, press-fit) was chosen for the crank journal for the first time on a Honda single-cylinder motorcycle. A cast-iron bush was selected for the crank bearing section in order to improve the rigidity of the crankcase housing and control changes crank journal's oil clearance arising from thermal expansion. In this way, the design takes productivity into account with an eye toward a global roll-out, while improving quietness at the same time.
Second, while using a built-up type crankshaft that allows the big end of the connecting rod to be used as a low-friction roller bearing, the optimal crank web shape was realized as a result of a computer analysis of strength and rigidity. This ensures high rigidity on a par with a solid type crankshaft.
Crankshaft rigidity is enhanced further and quietness is improved by placing the balancer's driving gear on the right cover inside the clutch housing -a design that narrows the distance between the left/right crank bearings and places a ball bearing at the tip of the crankshaft right side. For vibration, a primary balancer was chosen to produce a high-quality single-cylinder engine.
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| Cross-sectional view of crankshaft unit |
