Long-haul heavy-duty engines, presently almost entirely powered by diesel fuel, face challenges in meeting the worldwide need for greatly reducing their nitrogen oxides (NOx) emissions. These emissions produce smog and create respiratory problems. There is also a need to reduce greenhouse gas emissions. Dual-fuel gasoline-alcohol engine technology can provide a means to meet this need at an affordable cost. The engine could provide operation a wide fuel range from mainly gasoline use to 100% alcohol use. Use of stoichiometric operation and a three way catalytic converter can reduce NOx by around 90% relative to emissions from diesels with state of the art exhaust treatment. The alcohol fuel provides increased knock resistance, enabling high compression ratio, turbocharged operation that provides comparable efficiency and torque to a diesel engine. The alcohol can be ethanol or methanol and can be neat or a high concentration blend. It can also be in the form of a hydrous alcohol (alcohol and water). Use of hydrous alcohol can increase the fraction of fuel that is provided by gasoline. Use of hydrous alcohol instead of alcohol alone can reduce the amount and cost of the second fluid that is required and increase its availability. The use of alcohols with water eliminates freezing issues from use of water alone. Under some conditions it may also be possible to operate with only gasoline and water. We have used a computational model for knock to determine minimal alcohol requirements for needed for knock-free operation and to provide illustrative engine operation parameters for various forms of alcohol. Misfire limits are also taken into account. Dual fuel engines fueled by gasoline and alcohol could provide an attractive near-term, variable fuel approach for substantially improving air quality and reducing greenhouse gas when low carbon alcohols become widely available.