丰田油电混动普锐斯所搭载的先进电驱动技术在业界闻名遐迩，但一直以来，丰田都没有放弃对内燃机技术的探索。

在2018年的日内瓦车展上，丰田正式推出了全新“Dynamic Force”2.0 L四缸发动机系列。该系列将首推两款发动机：内燃机版和阿特金森循环混动升级版。丰田宣称，传统版和混动版的最高热效率将分别达到40%和41%。该发动机的详细信息及技术参数，您可查阅由丰田工程师在于2018年全球汽车年会（WCX2018）发表的相关论文，相关论文信息，请至正文最下方。

这两款发动机是丰田汽油机系列的最新力作。为使汽油机的制动热效率足以媲美轻型柴油机，丰田一直在积极探索，此前在2015年版的普锐斯上搭载了连续可变气门正时（VVT）1.8 L发动机和大容量废气再循环（EGR）系统，随后又在凯美瑞混动版中使用了2.5L四缸发动机。

与此同时，现代宣布伊兰特Eco车型所搭载的1.4L Kappa阿特金森循环四缸发动机和Ioniq混动版所搭载的1.6LKappa系列发动机的热效率也都达到了40%。

热发动机的热效率，指的是设备的有效输出能量和输入能量之比。以百分比计，热效率一定是在0%到100%之间。由于摩擦、热损失等原因，实际热效率通常远低于100%，汽车汽油机的热效率一般在25%左右。

此次丰田的成功并非得益于像马自达压燃SpCCI（火花控制压缩点火）发动机这样的突破性技术，而应该归功于丰田对消除一切不必要能源损失的执着。

一个重大的技术提升是新型发动机采用了全新激光熔覆阀座（laser-clad valve seat）。阀座和气阀表面无缝贴合，从而减少了阀座对进气直流的干扰，进而增强了燃烧室内的涡流。

为了匹配长冲程小孔径的设计（80.5 x 97.6mm），全新2.0 L四缸发动机增宽了气阀夹角，同时采用越来越普遍的四缸内燃机的气缸排量规格——496.5cc。

为使发动机的效率不受负荷和速度的影响，2.0L发动机搭载了雷克萨斯发动机的缸内直喷和进气道喷射相结合的双喷系统。

此外，新款发动机还采用了类似于马自达新一代高效直喷创驰蓝天汽油发动机（Skyactic-V）的超高压缩比，传统内燃机版为13:1，混动版为14:1。丰田表示，高压缩比、改进的高滚流进气口、双喷系统以及长冲程带来的高速进气，显著提高了Dynamic Force 发动机的燃烧速度。 

新款发动机沿用了雷克萨斯发动机的设计，以电子相位器取代了进气凸轮轴的液压执行器，进一步提高了可变气门正时系统的精准性。丰田研发副总Gerald Killman表示，“这样做的好处是发动机的响应会更快，特别是在寒冷的环境下。”汽油的黏性在低温下会升高，影响可变气门正时系统的响应速度。因此，电子进气凸轮相位器必不可少。”

Killman还表示，由于快速正时调校对排气的影响较小，因此排气凸轮作动器仍沿用液压式，“电子凸轮相位器成本更高，如果不能带来明显的效益，那为什么还要替换呢？”

为降低摩擦，新款发动机的活塞采用了树脂涂覆的激光菱形花纹侧裙设计。此外还搭载了可在各类环境下精准高效地控制冷却剂温度的电子调温器，以及只在需要的时候转动的电子水泵。

Killman表示，“暖机时不能有水流。”因此，和皮带驱动的水泵不同，当发动机温度较低时，电子水泵会自动关闭。

虽然油泵是机械驱动式，但其容量可变，因而可以减少寄生负荷。Killman强调，“我们需要改善每一个零部件。”

2.0L混动版在6600rpm时额定功率为126kW（170hp），在4800rpm时扭矩为 205 N·m（151磅英尺）。2.0L传统版发动机在6000rpm时额定功率为107kW（143hp），在4400rpm时扭矩为 205 N·m（151磅英尺）。

近来，丰田又推出了匹配新款发动机的全新CVT。该款由爱信精机生产的全新变速箱在车辆起步时采用齿轮传动，起步后改用皮带，传动比切换速度加快，更适合高速行驶。

Killman表示，这样的设计使CVT齿轮变速箱的响应速度更接近线性输出的齿轮传动，进一步增加了效率、提升了客户满意度，“新款CVT的设计初衷是希望在提高能效的同时带来更多驾驶乐趣。”

Killman还说，新款CVT传动皮带的角度由之前的11°收窄到了9°，换挡速度提升了20%，起步齿轮的应用使整体传动速度提升了15%。

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.

Author: Dan Carney
Source: SAE Automotive Engineering Magazine