The Ward’s 10 Best Engines competition has recognized outstanding powertrain achievement for 19 years. In this installment of the 2013 Behind the 10 Best Engines series, WardsAuto looks at the development of Honda’s naturally aspirated 2.4L I-4.
As more auto makers discontinue optional V-6s in favor of more powerful, often turbocharged I-4s, there's no shortage of excellent “base” 4-cyl. engines in the North American market.
This includes today's crowded and cutthroat midsize-sedan segment. Most of these vehicles use 2.4L or 2.5L engines generating between 170 hp and 200 hp.
But only one naturally aspirated I-4, Honda's new 2.4L gasoline direct-injection i-VTEC, standard in the ’13 Accord sedan and coupe, earns a 2013 Ward's 10 Best Engines award. It stands out from the rest of this hard-working pack in a number of ways, especially in refinement.
As the base powerplant in the ’13 Accord, it delivers 185 hp and 181 lb.-ft. (246 Nm) of torque, 4% more power and 12% more grunt than its predecessor. But the version that WardsAuto editors tested in the 6-speed-manual Accord Sport sedan, aided by a high-flow exhaust system, larger-diameter tubing and dual outlets for 52% more flow, ups that ante to a livelier 189 hp and 182 lb.-ft. (247 Nm).
Following 537 miles (864 km) of testing, editors report it delivered the happy combination of ample low-end torque, "wonderful" mid-range punch and "outstanding" high-rpm power while averaging an excellent 33 mpg (7.1 L/100 km). Then they laud it as "sophisticated beyond its price" and gush about how it "secures Honda's engine legacy."
As Honda's first U.S.-market DI engine, it can be mated with either the 6-speed manual or a new continuously variable transmission that also is a Honda first. Fuel economy of the standard version is 24/34 mpg (9.8-6.9 L/100 km) city/highway with the manual and a more impressive 27/36 mpg (8.7-6.5 L/100 km) with the CVT, while the higher-output Sport version tests the same with the manual and 1 mpg lower with the CVT. That's roughly on par with most competitors but slightly south of Mazda's new SkyActiv Mazda6 2.4L/6-speed auto and Nissan's Altima 2.5L I-4/CVT.
This new 2.4L 4-cyl. is the first production beneficiary of "Earth Dreams” technology, Honda's whimsical name for a set of next-generation technology advancements aimed at improving performance and efficiency. This includes gasoline DI, new-design combustion chambers, offset cylinder bores, a more efficient accessory belt design, a range of friction reducing technologies and a 3.5% reduction in engine weight thanks mostly to a lighter die-cast-aluminum block and a new resin cam cover.
"We are not only developing engines to improve fuel economy and comply with increasing awareness of environment impact," says Honda R&D Chief Engineer Toshihiro Akiwa via e-mail. "We also were aiming for major evolution in both fuel economy and dynamic performance by renovating the engine structure. This is the first attempt to apply ‘Earth Dreams’ technology with the goal to also satisfy Honda 'Joy to Drive' at a high level."
Key design objectives were achieving top-of-class fuel economy and acceleration, where the new CVT plays a major role, Akiwa says.
“(To do that) we reduced the structure weight and friction and adopted direct injection for better combustion performance to substantially improve fuel economy and torque," Akiwa says.
What were the most difficult challenges in achieving those objectives, and how were they overcome?
"An even fuel-air mixture in the cylinders is necessary to obtain stable combustion," he says. "That can be achieved with an intake port that forms a strong tumble, but that port design can also reduce the volume of air intake. So the challenge is creating the balance that achieves good tumble and high output.
"We did that by optimizing the contour of the port and the combustion chamber and combining that with VTEC variable valve-lift technology to realize both stable DI combustion and high output. Along with that, we also used CAE (computer simulation) to analyze the air-fuel ratio distribution and optimize the fuel-spray pattern and injection timing."
The lightweight pressure-cast aluminum cylinder head's exhaust ports are on its front side (vs. the previous engine's back-side ports) so the close-coupled catalytic converter can mount directly to the head without a separate exhaust manifold. The dual overhead cams (with improved journal finish to reduce friction) and four valves per cylinder are driven by a silent chain with a friction-reducing double-arm tensioner. A pair of chain-driven counter-rotating shafts in the oil pan take care of the second-order harmonic vibrations common to I-4 engines.
The cylinder head's new high-tumble intake-port design helps create both a homogenous fuel mixture for lower fuel consumption and high airflow for high power output. The combustion chamber shape and valve angles have been optimized with a narrower 35-degree angle (compared with the previous 51 degrees) between the intake and exhaust valves. This change decreases the surface-to-volume ratio and helps create a flatter, more compact combustion chamber that reduces unburned hydrocarbon emissions and enables a higher compression ratio of (11.1:1 compared with 10.5:1) without the need for premium fuel.
Another key contributor to higher output, fuel efficiency and low emissions is Honda's i-VTEC (intelligent Valve Timing and Lift Electronic Control). The system's "intelligence" lies in its ability to continuously vary intake-cam timing, based on input from sensors that monitor rpm, throttle opening, cam position and exhaust air-fuel ratio, relative to the exhaust cam.
This helps boost power and provides a smoother idle at lower speeds. Also at low engine speeds, i-VTEC optimizes intake valve timing and lift for rapid swirl-pattern cylinder filling. Then, as engine rpm builds past 4,800 rpm, it transitions to a high-lift, long-duration profile for superior high-rpm output.
The GDI system uses a compact high-pressure pump that allows both high fuel flow and pulsation suppression while variable pressure control optimizes operation of the injectors. A multi-hole injector delivers fuel directly into each cylinder to create the ideal fuel/air mixture with a stratified-charge effect: A richer mixture near the spark plug for easy ignition. This maximizes efficiency at lower power, helps cold-engine start-up and reduces emissions before normal operating temperature is reached.
To ease internal friction, the cylinder bores are offset from the crankshaft by 8.0 mm (0.31 in.) to give the connecting rods a more favorable angle, and the forged-steel crankshaft's journals are micropolished. The lightweight pistons' skirt design is optimized to minimize reciprocating weight and vibration, and their crowns are dished for more stable combustion and optimized fuel-charge distribution from the injectors.
A unique dot-pattern coating on the outer skirts of the pistons and 2-stage plateau cylinder-bore honing combine to lessen friction between the pistons and cylinders; the latter by creating an ultra-smooth surface that also decreases long-term wear. Friction is reduced further by the use of low-viscosity (0W-20) oil.
Finally, high-efficiency catalytic converters are key contributors to the engine's excellent emissions performance. The first mounts directly to the front of the cylinder head for fast activation after engine start-up, while a second converter is slightly downstream under the cabin floor. Both use a thin-wall design that increases internal reaction area and improves efficiency to help meet stringent California ULEV-2 and Federal Tier 2 Bin 5 certifications.
We asked Akiwa whether there is opportunity for future improvement to satisfy both customer and government demands. All he would say is Honda will keep working to "continuously improve fuel economy and performance."
