Engine manufacturers are adding electronic controls to their already-extensively redesigned diesels to meet increasingly strict exhaust-emissions limits. In 1993, federal EPA regulations significantly limited emission levels of particulates (the smoke and soot that's visible to the eye) and carbon monoxide and oxides of nitrogen (NOx). The 1998 federal regs zero in further on NOx, the smog-producing substance that is very difficult to limit without electronic engine controls. The strict '93 federal regulations forced design changes, making mechanical engines run far cleaner than earlier models. At that time, most engine builders had to redesign combustion chambers, pistons, turbochargers and other hardware. After 1993, most engine manufacturers started to include electronic controls. Virtually all 1998 truck diesel engines will have additional hardware and electronic changes. Vocational users, such as ready-mix producers, are loyal mechanical engine customers. Fleet managers have been comfortable with the mechanical engines' reliability, longevity and ease of maintenance. And better fuel economy isn't much of an issue in operations requiring extended idle periods and extended off-road driving. But engine builders say vocational buyers of electronically controlled diesels quickly learn to like the operational and maintenance advantages, as over-the-road buyers did beginning in the early 1990s. Mechanical engines are also less expensive. In most cases, a mechanical diesel still costs $2,000 to $3,000 less than an electronic model. But builders say the price differential is narrowing as increases in electronic-engine production reduce the cost of electronic components and declines in mechanical-engine production raise the per-unit cost of mechanical models. Tampering with electronic controls is more difficult than tampering with mechanical engine controls. Access to the engines' electronic control modules requires electronic tools or computers with special software as well as passwords. And most ECMs record maintenance time and date, making it easier to determine who altered the controls. Engine manufacturers must comply with limits on the total quantity of pollutants in emissions produced by their new engines. Caterpillar, Cummins and Mack will continue building mechanical engines into '98 by virtue of credits allowed by federal regulations. Builders earned credits by selling cleaner-operating engines sooner than required under clean-air regulations. The credits are used to continue producing and selling "dirtier" engines. These "dirtier" engines are designed specifically for vocational users like concrete producers. These engines include Caterpillar's popular 3306C, which may be offered well beyond 2000. Available for shorter periods will be Cummins' L10 and Mack's E7. However, Detroit Diesel's truck engines have all been electronically controlled for some time. And Navistar says it will be all-electronic in 1998. The article also includes several 1998 engine models, most of which are electronic.