GM's eighth-generation Ecotec 2.0T engine technology: How to turn a four-cylinder engine into a two-cylinder engine?

The most eye-catching feature of Cadillac's new compact SUV XT4 is its 2.0T variable-cylinder turbocharged engine, which is standard across the board. This engine is the first product of GM's eighth-generation Ecotec series engine. The most special feature of it is the application of Tripower variable valve management technology. Under certain working conditions, it can switch from four-cylinder power to two-cylinder power to achieve better fuel economy.

Four-cylinder engines with variable cylinder technology are relatively rare at present. Because of this, many people want to know how this engine's variable cylinder technology is achieved? Under what conditions will it become a two-cylinder operation? Is this technology mature and reliable? Therefore, in this technical analysis article, we will first answer these questions that everyone is concerned about.

How to realize variable cylinder technology?

The way GM's new 2.0T engine achieves cylinder change is similar to Audi's AVS and Honda's VTEC technology, which is to change the cylinder by switching cams of different sizes to change the lift of the valves. However, it can not only adjust the size of the valve opening, but also completely keep the valves of the two middle cylinders closed. When the lift of the intake and exhaust valves of cylinders 2 and 3 is zero at the same time, the two cylinders stop working. The principle is actually very simple, but it is not easy to implement.

On the intake camshaft, we can see that there are two cams with different lifts, high and low, at the 1st and 4th cylinder positions, which are used to change the size of the intake valve opening. In addition to the high and low lift cams, there is also a cam with no lift at the 2nd and 3rd cylinder positions. When switching to this cam with no lift, the valve will not be pushed open, that is, the intake valve is closed. This three-stage sliding camshaft is also the first structure of this engine, which has not been used on other engines before.

Since the exhaust side does not need to control the size of the exhaust valve opening, it only needs to close the exhaust valves of the two cylinders when the intake valves of the 2nd and 3rd cylinders are closed. Therefore, on the camshaft on the exhaust side, there are only two cams with different lifts at the 2nd and 3rd cylinder positions, namely the cam with lift and the cam without lift. Similarly, the exhaust valve can be closed when switching to the cam without lift.

The principle of switching cams in this engine is basically the same as Audi's AVS. The ejector pin on the top of the electromagnetic actuator is inserted into the spiral groove, and the lateral thrust generated by the rotation of the spiral structure is used to push the cam group to slide left and right, so as to use cams with different lifts to push the valve. There is a position sensor below the installation position of the electromagnetic actuator (the entire cylinder head is inverted in the figure), which is used to detect the position of the spiral groove. When it is detected that the position meets the ejector pin ejection condition, the ECU will send an execution command to the electromagnetic actuator to prevent the ejector pin from colliding with the spiral groove.

(The picture shows the spring limit structure of Audi AVS)

Since the cam group and camshaft are splined, in order to ensure that each cam can accurately align with the position of the valve roller rocker arm during sliding, similar to Audi's AVS, this engine also has a spring limit structure in the camshaft.

By applying Tripower variable valve management technology, this engine can achieve four-cylinder high-performance mode (when all four cylinders on the intake side use high-lift cams), four-cylinder economic mode (when all four cylinders on the intake side use low-lift cams) and two-cylinder super-economic mode (low-lift cams are used on the intake side of cylinders 1 and 4, and no-lift cams are used for the intake and exhaust of cylinders 2 and 3). When switching to the four-cylinder economic mode, the fuel economy of this engine can be improved by 5%, and when switching to the two-cylinder super-economic mode, the fuel economy can be improved by 15%.

Some people may wonder why the fuel economy is only improved by 15% when the four-cylinder engine is changed to two-cylinder engine, with half of the cylinders doing work? That is because even if the valves of cylinders 2 and 3 are closed, and no fuel is injected or work is done, the pistons of cylinders 2 and 3 are still driven by the crankshaft and continuously move in the cylinder, resulting in energy loss. At the same time, the energy consumption required for the movement of parts such as the crankshaft, camshaft, and flywheel has not decreased, so it is not easy to improve the fuel economy by 15%.

Under what working conditions does the cylinder change?

When will this engine use two cylinders? This question is actually difficult to answer, because the calibration of the engine is a very complex system engineering. Whether to switch to two-cylinder operation is related to factors such as the engine speed, torque, vehicle speed, gear, and throttle opening. It cannot be simply summarized as a certain speed at which it will switch to two cylinders, or a certain amount of throttle pressure to switch to two cylinders. From the official picture above, we can see that it is only possible to switch to two-cylinder operation mode when the engine speed is about 1200-2500rpm (no accurate data is provided) and the torque output by the engine is in the range of 0-60 Nm (no accurate data is provided). Note that it is only possible, not certain. 1200-2500rpm and 0-60 Nm are necessary but not sufficient conditions for switching to two-cylinder mode.

However, although the conditions for changing cylinders are very complicated, it does not mean that it is difficult or rare to change cylinders. According to engineers, this car can switch to two-cylinder operation when the speed is above 30km/h, the gear is in gear 3 or above, and the throttle opening is small (also a necessary but not sufficient condition). In the actual use of the car, such as driving at a constant speed on a flat road, accelerating slowly downhill, or slowing down when seeing a red light ahead, the engine may switch to two-cylinder operation, and the switching speed is very fast, there will be no setbacks, shaking or sudden weakening of power output, and the driver will not feel anything. In short, the ECU needs to consider many factors comprehensively to determine whether the engine can be switched to two cylinders to ensure that the power output in the two-cylinder state is sufficient to meet the driver's needs. It is worth noting that if the throttle is completely released, the engine will not switch to two-cylinder mode. The engineer said that there will be different fuel-saving strategies when the throttle is released, but the specific strategies were not detailed.

How reliable is the technology?

Adding such a complex motion structure to a high-speed engine will certainly make people worry about reliability issues. However, Audi's AVS technology, which has a similar spiral groove and sliding cam group structure, has been tested in the market for more than ten years, and the reliability of the structure is relatively strong. At the same time, Cadillac has already applied cylinder deactivation technology to the second-generation V8 engine of Safeway in 1980, so GM is very experienced in the strategic adjustment of variable cylinders. In addition, GM has an independent and well-equipped testing department in China. Before this new 2.0T variable cylinder engine was launched on the market, it had undergone a total of 9 types of system durability tests with a total of 4.65 million kilometers in China, including a new durability test specially added for the Tripower variable valve system, so there is no need to worry too much about the reliability of this new engine.

Other engine technologies

When this engine was first designed, fuel economy was given top priority, which is why variable cylinder technology was used. However, variable cylinder technology alone certainly cannot meet the requirements of increasingly stringent fuel consumption regulations, so on this new engine, we also see cylinder head integrated exhaust manifold technology to ensure that the engine can quickly reach the most efficient operating temperature when the engine is started from a cold start; 35MPa fuel direct injection system, compared with the previous generation of 20Mpa direct injection system, has higher injection pressure, better fuel atomization effect, and more complete combustion; fully variable displacement oil pump, which can adjust the power consumption of the oil pump according to the engine load to reduce energy loss; intelligent active electronically controlled carbon canister pump, which can actively recycle exhaust gas mixed with fuel so that it can quickly re-participate in combustion; composite oil pan, which further reduces the engine's own weight.

It should be noted that this new engine does not continue the center-mounted direct injection structure of the previous generation Ecotec engine, but uses a side-mounted direct injection structure, with the injector located on the side of the intake valve. This is because after adding a variable cylinder structure to the top of the engine, if the injector is placed in the middle of the top of the intake and exhaust valves, the entire engine will be very long and the top structure will be too complicated. However, the eighth-generation Ecotec engine will also launch models without variable cylinder technology, and it is still unclear whether they will all use the side-mounted injector structure.

In addition, due to the variable cylinder technology, when cylinders 2 and 3 are not working, the temperature of cylinders 1 and 4 will be significantly higher than that of cylinders 2 and 3, making the heat inside the engine uneven, which requires a more accurate engine temperature control system. To this end, this engine uses an ATM active thermal management system consisting of an electronic water pump and an electronically controlled ball valve module. Through various temperature sensors, it understands the actual temperature requirements of various parts, and intelligently and accurately controls the flow of each loop of the cooling system, so that the engine and transmission can operate within a stable and efficient temperature range.

Through this series of fuel-saving technologies, this engine is installed on the Cadillac XT4, with an official comprehensive fuel consumption of 7.2 liters per 100 kilometers, and can meet the National VI B emission standards in advance.

In terms of power improvement, this engine has not invested much new technology. After all, the power of the previous generation Ecotec engine is already strong enough. GM engineers also admitted that the new generation of engines does not intend to improve the power parameters, but will focus on improving emissions. Therefore, on this new 2.0T engine, we only see a relatively new power-enhancing technology, the twin-scroll, twin-volute turbocharger. This technology helps reduce exhaust interference and turbo lag, allowing the engine to reach peak torque at a lower speed. In addition, the use of an electronically controlled pressure relief valve effectively improves the power response of the engine at low speeds.

Finally, the maximum power of this engine is 177 kW, the maximum torque is 350 Nm, and the maximum torque output platform is 1500-4000 rpm. When it is installed on the XT4, the official 0-100 km/h acceleration performance is 7.6 seconds. Although the power parameters and acceleration performance are not as good as the 2.0T on the ATS-L, it is not easy to maintain such power while effectively reducing emissions.

It can be seen that under the pressure of increasingly stringent fuel consumption regulations in various countries, new engines now have to put fuel economy first, and even power has to be compromised. Manufacturers are no longer studying how to build a higher-performance engine, but trying to find a balance between fuel saving and performance. In such an environment, the performance of GM's eighth-generation Ecotec 2.0T engine is quite good. Judging from the official fuel consumption data, the application of fuel-saving technologies such as variable cylinders has greatly reduced emissions. From the actual test drive experience of XT4, its power performance has not been particularly affected, and it is still the most powerful in the same class.

However, the fuel consumption data and acceleration performance announced by the manufacturer are often obtained under relatively ideal working conditions. Whether this new engine can be as fuel-efficient as the manufacturer claims, we can only find out by getting a test drive car as soon as possible for performance testing.