Engine downsizing means to use a small engine that provides the power of a larger engine, through the use of recent technologies, e.g., a turbocharger or supercharger. It is the result of car manufacturers attempting to provide more fuel-efficient vehicles, often to deal with more stringent regulations.
The concept had been around for some time, though; Volkswagen is credited for the shift towards smaller engines in Europe with its Golf launched in the mid-2000s that introduced the TSI turbocharged petrol engine.
Meanwhile, domestic auto manufacturers lagged behind the trend.
It was Toyota that took the initiative, by developing a turbo engine, technology that they then put on hold for more than two decades.
It so happened one day in 2012 that Art Metal Mfg. was reached by Toyota, at the time, looking for a new piston for its turbo engines.
A piston is a core component of a reciprocating internal combustion engine.
The performance of a piston can be rated by seven elements that affect the engine as quoted:
“1. Output & Torque; 2. Fuel Consumption; 3. Emissions; 4. Reliability; 5. Noise and Vibration (N&V); 6. Cost; and, 7. Weight.
Meeting specifications/regulatory values, e.g., output, fuel efficiency, is just a fundamental requirement to do with exhaust performance.
A piston must be designed to micron-level precision in every aspect. A 10-gram increase in weight of a piston affects all of the other parts and, eventually, the weight of the engine may increase some kilograms.”
New engines are developed in large projects that, typically, take a minimum of three years and, if you count the initial phase, a total of five years.
In the early stages, auto manufacturers do not disclose models they plan to put a new engine in, to their suppliers. They specify the target power level of the engine.
The information that Toyota disclosed was: “It's for an in-line 4-cylinder turbo engine with a displacement of 1998 cc. The initial target output is 175 kW (238 PS).”
“To outperform competitors in all of the elements, including the specified output, was what they required of ART.”
That meant a piston capable of withstanding ever higher pressures and thermal loads.
The answer the project team came up with was to cast an wear-resistant ring made of Ni-resist cast iron alloy into the piston.
“ART had extensive expertise in casting different metals to aluminum alloys from their manufacturing of pistons for diesel engines.
We had never tried the technique with gasoline engines but knew, to top the competition, we had to in this case.”
An alloy containing iron is heavier than aluminum.
A piston is a component of reciprocating engines that slides up and down in the cylinder.
One thing to remember is that a gasoline engine can reach speeds of 6,000 rpm or over, much higher than that of a diesel engine.
The slightest increase in weight of a piston will affect all of the other engine parts.
To make it lighter, the initial drawing from the designer was based on a technique called “downgauging”, which removes the unnecessary portions of a component that have no effect on the performance of the piston.
The production engineer of the project took a look and found what was in it was something almost impossible unless a dedicated, new, and probably very expensive equipment would be allowed.
Downgauging an upper, lower, right, or left portion is a common technique, but taking out a diagonal portion in the location that is inside the component to be cast out seemed beyond the bounds of physical possibility.
Some may have seen a windup toy. Some automotive parts ART had produced utilized that concept of splitting one part into several sub-parts.
He recalls, “Let the mold have seven subsections, one of which, for instance, goes downward by 10 mm, then the rest moves diagonally. That works to cast a more complex geometry.
With improved structural and dimensional precision of the mechanism, the machines available there, without any change in machinery specifications, were good enough to deal with that elaborate geometry.”
Another challenge was there.
The piston head has three distinctive upright crescent walls.
They are designed to optimize the combustion when the fuel coming from the injector hits the walls.
The incline angle of zero degrees, i.e., upright, is important towards achieving the desired combustion efficiency.
“That was another challenge.
We needed a minimum 5˚ draft angle; otherwise, scoring would scar the wall surfaces.
But, from repeated simulations of prototypes, it turned out eliminating the incline in the walls would result in a significantly reduced rate of exhaust.
We wanted to be a part of a high-performance engine. So, we decided to go with zero degrees.”
He continued, “Machining that portion in a subsequent process was an option. But, to do that, at least another three machining centers were required.
That meant more cost and less productivity.”
Then again, they applied the technique of a “windup toy.”
They changed molds to draft subparts into the drawings. No change was needed for the cylinder cast machines.
It is quite common, in the early phases of engine development, that changes are made to bore diameters and combustion chamber geometries.
Changes in those, of course, lead to changes in piston design.
Production began 36 months after the project kicked off in full-scale, with all technical, engineering targets solved and target values achieved.
In July 2014, a new 8AR-FTS turbocharged engine with ART's pistons was mounted in the Lexus NX.
Right after, Toyota came to ART with a request: “We want to increase the output by another 5 kW.”
Toyota had a plan to launch subsequent models with higher power.
One of the project members said, “The new engine for which we did the best is now out there on the road. And now, we have to create another that outperforms that one.”
Initially targeted 175 kW (238 PS) was successfully achieved.
The next challenge was an additional 5 kW, i.e., 180 kW (245 PS).
“The current one withstands the required output but does not meet the design criteria.
The easiest way is to change the geometry of the piston, say, make it thicker, for increased mechanical strength. But, that would increase the weight of the whole engine.
Another option is to change the formulation of the aluminum alloy, though that would take too long. Given the time constraint, that seems unpractical.
Changes in the wall thickness and material components at the last minute can add another round of performance evaluations from scratch.”