Taming the emissions tiger

European regulations on emissions are being tightened. Contributing editor Michael Sibley looks at how manufacturers will build diesel engines to meet these new emissions criteria

Economical and emission free vehicles are the goals of every car manufacturer. They have to be: world-wide regulations demand no less. A long discussed and at last emerging technology for improving fuel combustion and economy is the common rail fuel injection system, now available on some diesel engines. Fuel is supplied to the injectors at constant high pressure from an adjacent gallery (the `common-rail’) and control over fuel quantity is exercised electronically at the injectors themselves from an engine management computer.

This contrasts with conventional practice where the injection timing and quantity is controlled by the pump, with the injectors only opening when the fuel pressure is high enough. The advantage is that fuel pressure, timing and quantity can all be varied independently to suit driving conditions. Very high fuel pressures are needed as is extremely precise manufacture especially for the small units required by passenger vehicle engines.

Petrol direct injection is moving forward to the front row from its earlier speciality use in motor racing. Toyota claims that its recently announced D-4 direct-injection engines achieve 30% better fuel economy than conventional engines, rivalling that of standard diesel engines.

The D-4 can run at ultra-lean air/fuel ratios of up to 50:1 using specially developed high pressure swirl injectors, helical intake ports, electronic swirl control valves and lipped combustion chambers inset into the piston crown. Very fast and precise injection, directed at the piston crown, is needed to run at these air/fuel ratios. The injectors operate at between 80bar and 130bar, about 40 times as high as conventional intake port injectors.

D-4 also has what Toyota calls intelligent variable valve timing to maximise torque in the low to mid-rpm ranges while maximising power at high rpms. The system continually varies the intake valve timing according to driving conditions. Combined with an electronic throttle valve to control the wide range of air-fuel ratios, the effect is to maximise fuel efficiency.

Part of Ford’s efforts to produce a high fuel economy, low emissions vehicle is centred on its research vehicle the P2000, (Figure 4). This is a lightweight, midsize car with a targeted fuel economy of up to 80mpg.

The first prototype, due later this year, uses lightweight materials as the key to high fuel efficiency making it 25% lighter than the steel Ford Taurus. Magnesium, carbon fibre, polycarbonates, titanium and metal matrix composites will be incorporated in the design.

Powered by a compression-ignition direct injection (CIDI) 1.2 litre engine, Ford claims it will be the lowest emission CIDI engine with the highest energy density and best fuel efficiency ever attempted by the motor industry.

Three types of powertrain will drive the P2000. The base model uses an automated version of a five-speed manual gearbox. A low storage requirement hybrid uses a small but high-power battery to store energy with an integral starter/alternator for quick restart. This allows the fuel to be optimised by shutting it down automatically during idling and deceleration. Some energy is recouped and stored by the starter/alternator during braking.

The third option, the post-transmission hybrid, includes an electric drive motor and has a much larger battery than normal with the capability to vary the drive between engine and electric motor depending on circumstances.

Engine cleanliness is being greatly improved by modern cylinder head designs and electronic management systems both designed to burn more efficiently. But there is a limit to what can be achieved by controlling emissions at source and post-combustion techniques, using catalytic converters, are needed to remove residual harmful emissions from the exhaust gases.

Catalytic converters were developed first in the UK in the 1970s to control vehicle emissions to meet USA and Japanese legislation, and have now spread around the world. Today, more than half of the worlds passenger cars have catalytic converters and more than 80% of new cars are built with them, including all new petrol cars in Europe since 1993.

Catalysts for petrol cars became possible with the advent of lead-free petrol, essential for them to function correctly. A well maintained unit will convert more than 90% of hydrocarbons (HC), carbon monoxide (CO) and nitrous oxides (NOx) into relatively harmless carbon dioxide, nitrogen and water, all present in the atmosphere we breath.

Diesel engine pollution presents a slightly different problem for the catalytic converter because of the presence of visible `smoke’ or particulates, but newer converters are tackling this additional aspect.

As regulations become tougher, improved ways of meeting them will be needed. One way will be to make catalysts better able to withstand high temperatures, enabling them to be placed closer to the engine. These `starter’ catalysts will reach operating temperature faster, tackling pollution during the critical warm-up period as the engine starts from cold.

Other developments under way include ways of using supplementary heat to bring the catalysts more rapidly up to temperature, using traps to store the polluting gases for later conversion when the catalyst is hot, and more efficient NOx removal from both diesel and lean-burn petrol engines.

Soon, emissions regulations will be at a level where no further economically viable improvements can be made. This is not to say that there will be no further work to do, but other factors will come into play.

The spotlight is being turned onto emission rates in terms of gm/km for each pollutant, and technology is being sought which will allow high engine efficiency at low emissions. Also, all manufacturers will be aiming to find cheaper, more efficient and more compact emission control equipment.

Then, as engine design changes, so do the needs of emission control systems. Lean-burn engines, for instance, are highly economical and produce low levels of HC and CO, but produce high levels of NOx. Toyota has a lean burn converter which absorbs NOx in a bed of rare earth during lean burn, converting it with a brief spell of rich burn running.

Cold starting is another problem. Some use electric heating or burning fuel to bring the catalyst rapidly up to temperature. Others move the catalyst closer to the engine to reduce warm-up time. Saab proposes to trap the first few minutes’ emissions in a container in the boot, burning them off when the system is up to temperature.

A newly introduced method of dealing with diesel exhausts combines a catalyst to remove 90% of the harmful gaseous emissions with a filter to catch the particulates. Designed by Johnson-Matthey, who pioneered the catalytic converter, the continuously regenerating trap (CRT) has two chambers, (see Design Engineering, March 1995). The first contains a matrix of fine ceramic channels coated with platinum which oxidises carbon monoxide and hydrocarbons into CO2 and water.

The catalyst also increases the proportion of nitrogen dioxide which Johnson Matthey discovered is the key to removing soot particles. So, in the second chamber the gases pass through a second matrix of tubes which are blocked forcing the gas to pass through the walls leaving the particulates on the tubes’ walls. Here the particles react with nitrogen dioxide where they are destroyed.

The CRT overcomes filter clogging because the particles are not retained. Another plus is that the new unit works at 250 degrees C, far cooler than the 600 degrees C needed to burn soot. No extra heating is needed and the single unit removes the majority of all harmful gases and particulates.

There are two drawbacks. The unit only works with very low sulphur diesel oil and cannot be retrofitted to any diesel engine. However, it is claimed to work well with new engines which are mostly turbo-charged.

Another way towards cleaner exhaust emissions is to burn compressed natural gas (CNG) instead of petrol or diesel oil, (see Design Engineering, June 1992). Its disadvantage is its relatively low energy density and the lack, at present, of a network of refilling stations. As a first step, Volvo have introduced a bi-fuel model, the S70/V70 designed to run on both petrol and CNG. Besides the normal petrol tank an 80l tank in the boot stores enough CNG for 150 miles of running. Running on gas is slightly more economical than petrol but there is a small loss of performance.

The driver can choose which fuel is used. Normally, gas is chosen for low pollution in city driving and petrol when out of range of the nearest gas filling station, of which there are 18 in the UK.

The engine has a uP-controlled fuel distributor supplying gas through injection valves to each of the five cylinders. Ignition is controlled for both fuels by the existing electronic system. If the gas supply is interrupted or runs low the engine reverts automatically to petrol power.

Another method for dealing with particulates from the `dirty’ diesel uses high energy electrons to `zap’ the noxious exhaust, reducing it to harmless water vapour, carbon dioxide and air. The aim is to bring diesel exhaust emissions to the high cleanliness levels achieved by petrol engines fitted with catalytic converters.

Still at an early stage, the device uses short bursts of high energy electrons to break down NOx and sooty particulates. A special electronic switch produces the high voltage electronic pulses at the rate of thousands of discharges per second.

These collide with the exhaust stream creating an ion plasma which reacts with the charged particles of nitrogen, oxygen and hydroxide to produce carbon dioxide, air and water vapour. Among groups working on this technology is one at the University of Southern California in Los Angeles.

For the future, engines will be sealed for life with no oil changes needed. The camless engine will have arrived and later on perhaps new engine types, hybrid cars and then the fuel cell.

Johnson Matthey’s patented Continuously Regenerating Trap (CRT) reduces emisssions of CO, HC, and particulate from heavy duty vehicle exhaust by up to 90%

Ford’s P2000 depends on an integrated systems approach to provide fuel-efficient, low emission transportation. It uses lightweight materials, an aerodynamic design and a sophisticated powertrain

{{Toyota (GB)Tel: Redhill (01737) 768585Enter 470}}

{{Johnson MattheyTel: 0171-269 8400Enter 471}}

{{Ricardo Consulting EngineersTel: Shoreham-by-Sea (01273) 455611Enter 472}}

{{Volvo Car UKTel: Marlow (01628) 422407Enter 473}}

{{Ford Motor CompanyTel: Brentwood (01277) 253000Enter 474}}

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