Toyota is well-regarded for its advances in electric drive technology with its hybrid-electric Prius models, but the company also continues to make progress with internal combustion engine technology.
At the 2018 Geneva Motor Show, Toyota engineers unveiled a new 2.0-L four-cylinder gasoline engine family it dubs “Dynamic Force.” There will be two versions initially—one for purely ICE-powered vehicles and a revised Atkinson-cycle version for hybrid-electric applications. Toyota claims the conventional version will achieve 40% peak thermal efficiency and the hybrid engine will reach 41%. Toyota engineers will explain the engines in further detail in an SAE Technical Paper to be presented at WCX 2018.
These are the latest Toyota gasoline ICEs to offer levels of brake thermal efficiency (BTE) approaching that of light-duty diesels. They follow a 1.8L VVT in the 2015 Prius that used a large-volume exhaust gas recirculation (EGR) system, and the 2.5-L four used in the Camry Hybrid.
Hyundai also claims 40% BTE for its Kappa-family 1.4-L Atkinson cycle four used in the Elantra Eco and the 1.6L Kappa used in the Ioniq hybrid.
A heat engine’s thermal efficiency is the ratio between the useful output of a device and the input, in energy terms. The thermal efficiency must be between 0% and 100% when expressed as a percentage. Due to factors including friction, heat loss, etc., thermal efficiencies typically are much less than 100%. A typical automotive gasoline ICE operates at around 25%.
Toyota’s recent success is achieved, not by breakthrough like Mazda’s gasoline-compression-ignition SpCCI engine, but by relentless nibbling away at waste.
A significant improvement is the use of a new laser-clad valve seat that shrinks the seat to the absolute minimum of the contact surface with the valve face. This reduces the seat’s interference with the intake port’s straight shot into the combustion chamber, which contributes to intake charge swirl inside the combustion chamber.
The valves in the new 2.0-L four also are arranged in a wider included angle to fit within the smaller diameter of the undersquare (80.5 x 97.6-mm) bore and stroke design. Each cylinder measures 496.5cc, increasingly typical for current generation 4-cylinder ICEs.
The engine features a dual fuel injection system, as seen previously on the Lexus-brand engines, with both direct injectors and port injectors to provide the best efficiency under all loads and engine speeds.
Very high compression ratios—13:1 for the conventional engine and 14:1 for the hybrid—are similar to those used by Mazda for its Skyactiv-G. Combined with the revised tumble-inducing intake ports, and dual-injection scheme and high intake charge velocity associated with long-stroke designs, the Dynamic Force Engine boasts much faster combustion, according to the company.
Increased variable-valve-timing control precision is provided by electronic phasers for the intake camshafts. These replace hydraulic actuators, as Toyota has also done on Lexus engines. “The advantage is that it is faster, especially in cold conditions,” explained Gerald Killman, vice president of research and development. Cold, thick oil leads to sluggish variable valve timing, so the electric intake cam phasers are critical.
Exhaust cam actuators remain hydraulic because the exhaust side is less sensitive to rapid timing adjustments, he continued. “[Electric cam phasers] cost more, so if there isn’t a clear benefit, why bother?” Killman asked.
Low-friction pistons feature laser-crosshatching in their resin-coated skirts for reduced friction. Other technologies aimed at higher efficiency include the use of an electronic thermostat to precisely control the coolant temperature for efficiency under all conditions, and an electric water pump ensures that the water pump only spins quickly when it needs to.
“In the warm up, you want to avoid any water flow,” said Killman. So unlike a belt-driven pump, the electric pump just turns off when the engine is cold.
The oil pump is mechanically driven, but it is variable capacity, so its parasitic load is reduced. “We need improvement in every single item,” Killman emphasized.
Power ratings for the conventional 2.0-L engine are 126 kW (170 hp) at 6600 rpm and 205 N·m (151 lb·ft) at 4800 rpm. The new hybrid engine is rated at 107 kW (143 hp) at 6000 rpm and 180 N·m (132 lb·ft) at 4400 rpm.
Toyota also recently announced an innovation for the CVT to be matched to the engine. This Aisin AW-supplied transmission will employ a conventional first gear ratio for launch, then switch to the CVT when it upshifts from first gear. This lets the CVT shift its ratios to higher-speed driving.
The result is improved efficiency and better customer satisfaction because the response is more like the familiar linearity of a geared transmission, reported Killman. “This new CVT was designed looking to driving pleasure as well as efficiency,” he said.
Shift speed is a claimed 20% quicker because the CVT’s belt angle is 9° rather than the previous 11°, and the transmission’s overall ratio spread is 15% greater thanks to the addition of the launch gear, Killman explained.Continue reading »