The development trends of recent years toward higher specific engine output have led to the gas exchange valves being subjected to increasing thermal loads. The component temperatures at the exhaust valve are already above 800°C in many cases, which is a challenge for typical high-strength valve materials. Combustion optimization by thermal management of the valves also plays an increasing role.
Typical limitations in terms of operational reliability of λ = 1 concepts are the maximum temperature at the exhaust valve, the turbine inlet temperature, and/or the maximum permissible inlet temperature of the exhaust gas at the first catalytic converter. In order to protect the components, enrichment of the mixture is often necessary at high engine speeds and loads to limit the maximum exhaust gas temperature and prevent the components from being thermally overloaded. Currently, the knock limit additionally presents a substantial obstacle to optimizing fuel consumption in gasoline downsizing engines. The “knocking” phenomenon is critical to the further development of CO2-optimized gasoline engines.
MAHLE has developed a new technology on the basis of a conventional hollow valve with cylindrical bore, whereby an extended hollow cavity can be made in the valve head with just one additional process: electrochemical machining (ECM). This enlarged hollow space allows liquid sodium to dissipate the heat during engine operation even better. The valve head is cooled by the shaker effect: the hollow space is not completely filled with sodium, so that it is shaken as the valve moves; the sodium breaks down and thereby produces significantly greater heat dissipation via the valve guide.
This technology can be used for both intake and exhaust valves. The surfaces in the combustion chamber are cooler as a result (reduction of approx. 30 to 50 K), allowing the knock limit to be shifted and thus enabling a more optimal selection of the firing point in the design of the gasoline engine cycle. The goal is to achieve a reduction in fuel consumption. The valve mass is likewise reduced by 3 to 6 percent compared with conventional hollow valves. Measurements substantiate the temperature reduction on the exhaust side. An internal high-stress endurance test to establish service life has also been concluded positively. The EvoTherm valve is an important addition to the MAHLE product portfolio. The necessity of applying such technologies is already clearly evident today.
MAHLE takes a globally leading position in the area of hollow valves, with decades of experience. The MAHLE EvoTherm valve is the latest development to support engine manufacturers even more fully as they confront current and future challenges in engine development. Because the EvoTherm valve is based on a conventional hollow valve, it can be considered a low-cost solution.
Beyond the potential of the EvoTherm valve, the thermodynamic advantages and industrial feasibility of the MAHLE TopTherm valve are now being evaluated. This valve concept is designed as a lightweight valve, and its advantages have been proven with respect to reduced friction in the valve train, which can lead to a reduction in consumption in the NEDC by up to 0.5 percent. At the same time, the composite valve offers more extensive potential for reducing temperatures at the thermally highly loaded component surfaces due to its construction as a rigid, rotation-symmetrical surface structure with a large sodium-filled cavity. Based on thermal simulations and temperature measurements, there seems to be a potential to reduce the disk temperature by significantly more than 100 K.
In addition, this concept provides opportunities for the reduction of component protection measures when used as an exhaust valve, due to the more uniform component temperature distribution and its proven high thermal load capacity. While the maximum permissible temperature of the exhaust valve is typically a limiting factor, besides the intake temperature at both the catalytic converter and the turbocharger, the MAHLE TopTherm can be designed even closer to the target of λ = 1. In comparative tests with conventional solid and hollow valves, the composite hollow valve demonstrated significant advantages in fuel consumption in the full-load range while maintaining the maximum permissible intake temperatures at the catalytic converter and turbocharger. These are a result of the shifted knock limit, with the resulting improvement in the center of combustion mass and leaner full-load design. Good potential is evident in the transient range as well, because the shifted knock limit allows a higher basic compression ratio and comparable performance targets can be achieved with lower boost pressure. A study of the industrial feasibility of this challenging valve technology with regard to production is nearly complete.
