Diesels have long had direct injection. In fact, their latest iterations feature multiple pre-, primary and post-injections for improved economy, emissions and power.
Our Data Panel gives a gasoline engine's Recommended fuel, as cited by the auto­maker. There is a relationship between compression ratio of a gasoline engine and its fuel's octane requirement; similarly, of a diesel engine's compression ratio and its cetane requirement.
High-performance gasoline engines are designed to run on premium fuel, typically of 91-pump octane (the number cited on the filling station pump). Others are designed for less expensive regular 87-octane. All automotive fuels these days are unleaded, no longer using tetraethyl lead as an octane enhancer.
Flex-fuel cars are capable of running on gasoline or E85 (a blend of 85-percent ethanol/15-percent gasoline) or any combination of the two. Their diminished E85 mpg reflects ethanol's relative deficit of energy.
Octane is a measure of a gasoline's resistance to knock or detonation, uncontrolled ignition of the fuel/air mixture at the wrong time and place within the combustion chamber.
Modern engines have knock sensor systems that adjust ignition timing to avoid this harmful phenomenon. An engine tuned for premium will run on regular fuel, albeit with a loss of efficiency and power (that may or may not be perceptible). There's no benefit in octane higher than the recommended one (as systems generally adjust downward in response to knock, not upward until they hear it).
Catalytic converters control the three regulated automotive emissions, HC (hydrocarbons), CO (carbon monoxide) and NOX (oxides of nitrogen). As these emissions-control system are fitted to all cars these days, we don't cite the actual hardware at the moment.
Diesels have special needs for controlling their NOX and particulates to U.S. limits (Unlike those in Europe, our regulations are the same for gasoline and diesel engines.) Some diesels require urea injection to meet our NOX regulation; others potentially do not. As urea systems require periodic replenishment (at oil-change intervals), they may earn a Data Panel mention before long.
Also, a word on CO2, a hot-button issue for many. Carbon dioxide is part of our every exhalation; it's also a product of any carbonaceous combustion. With regard to the automobile, its output is directly related to fuel consumption: The more carbonaceous fuel burned; the more CO2 produced. Reduced CO2 comes with enhanced fuel efficiency; there is no catalytic converter that treats it.
Evolving technology Evolving Terminology
Hybrids have both a conventional engine and electric motor. Their enhanced efficiency comes from the interaction of these two power sources. A series hybrid uses its engine solely to run a generator supporting its battery and electric propulsion. A parallel hybrid may be either engine- or electric-motor-driven or both. A parallel hybrid (such as the Toyota Prius or GM 2-Mode) also has modes of series operation. By contrast, a series hybrid's engine never provides propulsion directly. Though GM prefers to call the Chevrolet Volt a "range-assisted electric vehicle," by our definition it's a series hybrid.
Thus far, there are three degrees of hybridization. A full hybrid (think Prius or Volt) has a mode of purely electric propulsion. A mild hybrid (think the upcoming Honda Insight) offers modes of significantly enhanced electric performance, but its engine still provides primary propulsion. R&T makes an added distinction in a microhybrid (the current Chevrolet Malibu is exemplary), where an oversize alternator supports start-stop operation but is designed to provide only modest performance enhancement.
What of plug-in electrics, pure electrics and fuel cells? They're coming (sooner than some may think) and our Data Panel will be ready for them.