Have pistons, will travel
Text: Jake Venter
Source: This article is taken from the September 2011 issue of CAR Magazine.
Engine development during the next 15 years.
The Bosch predictions
- By 2020, the annual demand for cars and small trucks will reach 103 million units.
- Only three million will be all-electric or plug-in hybrids.
- Six million will be conventional petrol or diesel hybrids.
- This leaves a good 100 million vehicles powered, even if only partly, by internal-combustion engines.
Note: last year, 71 million cars were sold. This implies the market for piston-engine cars will grow by 40 per cent over the next 10 years.
Some people are convinced that, 10 to 15 years from now, we’ll all drive plug-in electrics or hybrids. But, for every person who’s excited about this prospect, 10 are scared it will happen. The characterless whoosh of an electric motor simply cannot excite the senses like the throb of piston-driven exhaust pulses.
For this paranoid majority, Bosch, the largest suppliers of OE-parts in the world based on 2010 sales figures, has good news. The company’s engineering and marketing gurus have been studying trends and realities, and they’ve come out with the prediction that, by 2020, there will annually still be a market for 100 million internal-combustion engines (IC-engines).
Reaching C02 targets.
In Europe, the biggest driving force for the shift to plug-ins and hybrids is the ever-tightening C02 targets set by the EU, as shown in the accompanying panel. In Europe, the contrast between C02 levels for IC-engines and electric power units is sharper than it is in most other countries, because most of the continent’s energy comes from clean sources.
In South Africa and many other countries with mainly coal-fuelled power stations, the contrast is blurred. It has been calculated that, in these countries, a plug-in electric vehicle will still pollute at the equivalent of 130 g/km owing to the fact that coal is practically all carbon, hence a coal fire produces more carbon dioxide than any other fire. This figure is considerably higher than the emissions an average 1,1-litre petrol engine produces.
The key to reaching long-term C02 targets lies in eventually producing more diesel engines, as well as electric vehicles. The former produce less C02 than petrol engines, as measured in g/km. In the medium-term, the efficiency of both types of IC-engines must be improved even further.
Bosch predicts fuel consumption and C02 values of IC-engines will drop by 30 per cent in the medium-term, while the changes made by automakers to the rest of their cars will result in emissions reducing by another 20 per cent. These changes include low-resistance tyres, lightweight construction and overall drag reduction. This means that the targets for 2025 are reachable.
Turbocharging is preferred, because the impeller is energised by the exhaust gas
Determining CO2 values
For the NEDC test (New European Driving Cycle, see panel), the total amount of exhaust gas emitted is collected and chemically analysed.
These results are then used to calculate the amount of C02 in the exhaust. In theory, there is close link between fuel consumption and C02 emissions, so that one can be calculated from the other.
In practice, this means that anybody can calculate their car’s C02 values.
Simply multiply the fuel consumption in litres/100 km by 23,3 for petrol and 26,2 for diesel engines to determine C02 emissions in g/km.
These values change slightly for different fuel compositions and fuel densities, but they’re a good average for South Africa.
A number of models from Volkswagen, Volvo and BMW are already complying with the 2015 CO targets, while other models will soon follow.
Petrol hybrids have consumption values close to those of comparable diesels. And Peugeot’s new 3008 diesel hybrid is expected to already comply with the C02 targets for 2020.
Reducing consumption by downsizing
One of the most successful ways to reduce fuel consumption is to reduce an engine’s displacement. Many of the latest model ranges include downsized engines in their line-ups.
Currently, downsizing occurs in two ways. Mercedes-Benz, for example, sticks to a fairly small displacement of 1 796 cm3 for its recently updated C180, C200, and C250 models, while increasing the outputs of the more expensive variants by increasing the turbo boost.
Most other manufacturers of engines in the 1,6- to 1,8-litre class are designing or are already marketing engines with a reduced number of cylinders. When a four-cylinder engine is compared with a three-cylinder unit the loss of one main and one con-rod bearing, together with the loss of a con-rod, piston and rings, as well as the reduced number of valves and valve gear, is claimed to improve fuel consumption by up to 20 per cent.
The loss of performance that would accompany a reduction in displacement is then usually restored, and sometimes even bettered, by employing a supercharger or a turbocharger to get more mixture into the engine. Turbocharging is preferred because the impeller is energised by exhaust gasses, instead of being engine-driven, so it uses energy that would otherwise go to waste.
Improving petrol engines
Direct injection
One of the most significant developments is the adoption of direct fuel injection. This is already a feature of many modern engines and replaces the in-manifold fuel injection that has been employed up to now. Fuel evaporation inside the cylinder helps to cool the combustion chambers to allow higher compression ratios to be employed in combination with turbocharging without inducing the detonation that usually accompanies an excessive rise in combustion heat.
Lean-burn
An even more important advantage is that direct injection, in combination with specially shaped piston domes, is being utilised to achieve an area of slightly rich mixture surrounding the spark plug. The rest of the combustion chamber is filled with a leaner mixture. This process, known as lean-burn, results in a fuel consumption improvement.
In 2007, Mercedes showed a concept car fitted with a Diesotto engine. This takes the lean-burn combustion process further. The engine runs like an Otto-cycle (petrol) engine at large throttle openings but reverts to a sparkless diesel cycle at smaller throttle openings. This combines the economy of a diesel engine with the power-per-litre capability of a petrol engine. The company claims this engine will be production-ready in four years.
Very high compression ratios
Another promising development is the imminent release of very high (14:1) compression ratio petrol engines by Mazda. These engines sidestep the detonation problems resulting from combining turbocharging with a high compression ratio by employing the Miller cycle. This is similar to the familiar four-stroke Otto cycle but, on the compression stroke, the intake valve closure is delayed to such an extent that the compression process takes place only during the last 70 per cent or so of the compression cycle. This means that some of the intake mixture is forced back into the intake manifold to reduce pumping losses. The actual compression ratio is also far less than the nominal 14:1 value. A turbo is used to compensate for the resultant power loss.
The advantages are a reduction in combustion temperature, a diminishing possibility of detonation occurring and higher energy-extraction from the hot gasses because of the high expansion ratio that results from the full 14:1 compression ratio that applies on the exhaust stroke.
Reducing other losses
The reduction in losses incurred by engine accessories is another success story. Many water pumps are now demand-driven, i.e. they’re electronically controlled to pump only as much cooling fluid as is needed in any particular situation. Multi-stage oil pumps, are also coming into fashion. They deliver maximum pressure only when the engine is working hard. The latest alternators primarily charge the battery when coasting so that very little engine power is wasted in performing this duty.
Stop-start systems are fast gaining ground. A beefed-up starter motor is used together with the electronic-control systems to ensure that the engine stops when the car is stationary for a few seconds and seamlessly starts again when the clutch or throttle is activated. It is claimed that fuel savings as high as 8,5 per cent can be achieved in heavy traffic.
Higher injector pressures will enable smaller-diameter injector holes
Improving diesel engines
Downsizing has found its way into diesel engines, too. In this case, just about the only way to restore the lost power is to increase turbocharger pressure, but this is done only if the fuel-injection pressure is also increased. Bosch engineers have increased the injection pressure in its units from 1 000 bar to 2 000 bar over the years. Later this year, the company will start to produce 2 200-bar common-rail systems for passenger cars, and are working on 2 500-bar systems for implementation in a few years’ time.
Higher injector pressures will enable smaller-diameter injector holes to be provided, with the result that fuel-droplet size can be reduced. This speeds up the combustion process and increases combustion control, especially when used in combination with multiple pre- and post-injections per cycle. The result is an improvement in fuel consumption and a reduction in nitrogen-oxide exhaust emissions.
Future developments
Engineers are working on more comprehensive variable valve control systems, even for diesel engines.
A number of experimental engines are employing solenoid-operated valve gear that promise completely variable valve-lift and timing.
Apparently, the biggest problem with such a desirable layout is that the solenoids are bulky and noisy.
More precise combustion control is also on the cards. Some production engines are already using spark plugs as combustion-quality sensors, while maximum-combustion-pressure sensors will soon be in production.
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