The US State of California is the leader in the field of emission legislation. Its "LEV" (Low Emission Vehicles) requirement, roughly equals to Euro 2000, will be effective in 2000. 3 years later, "ULEV" (Ultra Low Emission Vehicles) requirement will restrict the pollution level to 30% of today's standard, that is similar to the Euro 2005.
At the focus of attention is the so-called "non-methane organic gases" (NMOG) - organic hydrocarbon compounds such as aldehydes, alcohols, alkanes, aromatic compounds and esters found in car exhaust, and which experts consider to be responsible for the increase in the concentration of ozone in the atmosphere. All car makers are required to ensure that the passenger cars which they sell in California do not exceed a certain annual NMOG fleet average.
Honda's leading ULEV and ZLEV technology
Honda is currently leading LEV and ULEV technology. Back in 1995, it created the first ULEV engine in the world and installed to Accord. Today, while other car makers are working hard on their ULEV engines, Honda once again lead this field by introducing an even cleaner ZLEV ( "Zero" Low Emission Vehicles ) engine.
Basically, ZLEV based
ULEV but improves the catalytic converter arrangement. Since I only got
the pictures of ZLEV, let me explain its theory first and by the way
emission by three stages :
|Mazda's 2.3 litres Miller Cycle engine is the only one of its kind. Although it achieved 10 - 15 % fuel consumption reduction over comparable coventional engines, high production cost prevent it from being popular.|
In Mazda's Miller Cycle V6 engine, inlet valves close at 47 degrees after BDC (bottom dead center, ie, the lowest position of piston during a cycle). This equals to 20% of the height of stroke. In other words, during the first 20% of the compression stroke, the intake valves remain opening, thus air flows out without compression. Real compression activated during the remaining 80% stroke. Therefore, the real effective capacity of the engine is only 80% of the volume of combustion chamber. Compression ratio is decreased from 10 : 1 to slightly under 8 : 1.
Valve timing of the Miller Cycle V6
Until now, you probably still don't understand its objective. Be patient, I am going to explain now.
Lower compression ratio means less energy loss in compressing air, i.e., the so-called "pumping loss". Moreover, lighter compression leads to lower temperature, thus reduces heat loss in cylinder wall and pistons. To compensate the reduction in real capacity, a supercharger is employed to increase the air density such that the engine actually resume 100% capacity. Of course, the supercharger must generates less pumping loss than those gain by reducing compression ratio. Otherwise Miller Cycle engine will be no more efficient than ordinary engines.
Note that the expansion stroke is the same as ordinary engines, it is not reduced like the compression stroke. As a result, power delivery and is as smooth as normally aspirated engines.
Mazda's Miller Cycle engine burns 13% less fuel than its 3 litres conventional sister engine. It also generates more power and better torque curve. However, since its introduction in 1994 until now, no other car makers follow its trend. Even Mazda itself did not produce another Miller Cycle engine. Why ?
Think about it: although it is claimed to be a 2.3-litre engine, it is actually constructed as a 3-litre engine, no matter in dimensions and in material. Then, the supercharger and twin intercoolers (one per cylinder bank) will be extra cost compare with conventional 3-litre engine.
For a V6, this might be forgiveable, but those additional cost will be relatively expensive for a low cost four-cylinder engine. As a result, Miller Cycle concept can hardly be popular in the market.
To understand how throttleless engine save fuel, we must first know how our conventional engines work. When you floor down throttle pedal, you do not control the amount of fuel injection (which would have led to change of air-fuel mixture ratio hence misfire) but the amount of air running into the cylinders. Fuel injection system monitor the amount of air passing the throttle butterfly and then determine the corresponding amount of fuel needed. The more the throttle butterfly opens, the more air runs into the combustion chamber. This is how your throttle pedal control the engine. The below picture shows a conventional engine with throttle butterfly in the intake manifold.
rid of throttle butterfly thus saves fuel, especially at lower rev.
to the EU combined test cycle consist of high speed and low speed
the 1.8-litre Valvetronic engine used by 316ti Compact saves 10% fuel
53.3 mpg or 5.3 litre / 100 km). For those who regularly drive in city,
the saving could be bigger. Moreover, unlike direct-injection (the most
popular fuel-saving technology right now), it need not to drink
Now you might think about Honda's VTEC. Can Valvetronic use its variable lift to increase power like VTEC ? Sadly, Valvetronic is actually less efficient at high rev than conventional engines, let alone VTEC. As you can see, the camshaft drives the long intermediate rocker arms, in turn drive finger followers, this generates quite a lot of friction. Therefore the efficiency and refinement of Valvetronic engines drop rapidly at over 6,000 rpm. No wonder in the foreseeing future BMW will not equip its M-power engines with Valvetronic.
The answer is: not every one like this kind of cruising. If you drive in a hurry, you don’t like to reduce the engine speed as you want to accelerate as soon as overtaking opportunity comes. If you drive in traffic, which calls for intermittent acceleration and deceleration, you are not likely to select the 4th and 5th gear too. A considerable large portion of our daily driving is spent on the "low gear, high rev" pattern which does not optimize fuel consumption. This makes EGR worthy.
EGR recirculate some of the exhaust gas (probably up to 10%) back to the inlet valve via a recirculation pipe. The amount is determined by engine ECU and controlled by a valve at the recirculation pipe. When the ECU believes the engine is running at light load, it directs the exhaust gas back to the combustion chamber. As the exhaust gas is largely non-ignitable and takes no part in the combustion process, the fuel injection can emit less fuel without worrying failure combustion. As a result, the engine still run at roughly the same speed but power output, fuel consumption and emission are all reduced.
Yet this is only half of the story. Another large portion of the fuel and emission saved is contributed by the reduction of pumping loss. Pumping loss is the power used to pump the fresh air into the cylinder and pump the exhaust gas out, of course the "pump" is the piston. To reduce the power wasted in pumping in fresh air, we can pressurised the input air, that is, using turbocharger, or alternatively add some high-pressure, speedy gas into the input stream. In our case of EGR such gas is the exhaust gas.
To our environment, apart from the emission reduced via consuming less fuel, EGR also reduces NOx emission. How ? as the amount of combustible gas is reduced, temperature in combustion chamber is also reduced. Because NOx generates at high temperature, NOx emission is also reduced.
Related topic : EGR by means of VVT