Using hydraulic actuation effected by oil pressure, Honda came up with a system that would switch between high and low valve lift levels using two cam profiles and two rocker arms per cylinder. The switch is made using hydraulic pressure to push/release a sliding pin that locks or unlocks the bigger and the smaller rocker arms.
At low engine speeds, the pin is retracted, disengaging the bigger rocker arm. The valves are operated by the low-profile cams, enabling low valve lift. At higher engine speeds, increased hydraulic pressure pushes the pin, engaging the bigger rocker arm. The valves are now operated by the high profile cam, allowing high valve lift.
Switching between the two is actuated via a solenoid valve and controlled by the ECU, which takes account of engine oil pressure, engine temperature, vehicle speed, engine speed, and throttle position. The switchover point is variable, between a minimum and maximum point, and determined by engine load. The switch back from high- to low-rpm cams is set to occur at a lower engine speed than the upswitch to avoid a situation in which the engine is asked to operate continuously at or around the switchover point.
Today, most other manufacturers have followed Honda’s example and call their adaptation of VTEC by many other names – VVTi and VTVT are ones you might recognise.
A unit injector is a combined pump and injector – think miniature bicycle pump filled with liquid. Imagine pinching the outlet nozzle with your fingers, pushing the plunger in to pressurize the liquid, then releasing your grip on the tube, allowing the fluid to escape. The unit injector works in a similar way and on Volkswagen’s two-valve diesels a single camshaft operates both the inlet and exhaust valves as well as the unit injectors.
The main benefits of unit injectors are high pressure for power, efficiency and accuracy. On the downside, cylinder heads must be designed from scratch to accommodate the relatively bulky injectors and lack flexibility. In a common rail there is more freedom in the injection timing.
The common rail is a large pipe or manifold, continuously fed fuel under pressure by an engine-driven pump. Injectors have no pumping ability and are fed by pipes leading from the rail, fuel being injected whenever it is required by opening the injector electronically. Just think of holding your thumb over the end of a pressurised hosepipe then letting go. Here injection timing is mechanically independent of the engine cycle.
Pilot injections prior to the main one reduce noise, while post injections can raise exhaust temperature to heat up particulate filters. Common rails sound quieter than the harsher sounding Pumpe Düse diesels, especially at idle. The downside is that injection pressure is not as high as in a Pumpe Düse engine.
Variable Geometry Turbocharger
A turbocharger, or turbo, is a mechanical device that increases the pressure of a gas by reducing its volume. A turbocharger is used for forced induction of an internal combustion engine. A turbocharger increases the density of air entering the engine which results in creation of more power. Variable geometry turbocharger (VGTs) is a form of turbocharger.
VGT: How does It Work
VGT allows the effective alteration of the aspect ratio (the ratio of longer dimension to the shorter dimension) depending upon the engine conditions. The optimum aspect ratio at low engine speeds is very different from that at high engine speeds. At low speeds if the aspect ratio is too large, turbo will fail to create boost. Similarly at high speeds if the aspect ratio is too small, the turbo will choke the engine leading to high exhaust pressures. The pistons have to work harder to suck in the fuel-air mixture which results in low intake-manifold pressure and ultimately lower power output. Maximum efficiency can be delivered if the geometry of the turbine is altered considering the aspect ratio. Thus, the VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds. This is the reason why some of the configurations of VGTs do not even require a waste gate which is responsible for diverting the exhaust gases away from the turbine wheel.
In diesel engines, a VGT is responsible for controlling boost at lower speeds. The VGT has a set of movable vanes in the turbine housing, and they control boost by controlling exhaust turbine inlet pressure mainly at high speeds. At low engine speeds when exhaust flow is low, the vanes are partially closed. This increases the pressure of the exhaust pushing against the turbine blades, making the turbine spin faster and generating more boost. They provide big improvements in diesel engine efficiency and emissions.