DIESEL-POWERED cars, trucks and buses are on the way. The promise of their greater fuel economy--about 25 percent better than current gasoline engines--is too good to ignore. Projections are that by the end of the decade diesels will account for one in every four new cars and two out of three new trucks and buses.

But diesels also raise a serious new air pollution controversy. Their exhaust contains dozens of chemicals including known and suspected carcinogens, toxic substances and mutagens. Diesels also emit about 50 times more particulates than do gasoline engines. These tiny carbon particles are so small that they escape the body's normal respiratory defenses and are carried deep into the lungs, where they lodge and may remain for years. And they carry the dangerous chemicals with them.

But though there is reason to be concerned about the health hazards of diesel exhaust, there is no conclusive proof of harm. The laboratory tests raise danger signals, but the effects have not been demonstrated in humans. The National Academy of Sciences, evaluating the situation for the government, listed dozens of research projects that will take years to complete before definitive answers are available.

How then should diesels be regulated? The choice, as it so often seems to be these days, is between two possible errors: risk the costs of what may prove to be unnecessary controls or risk increased cancer, respiratory disease and other possibilities because of current uncertainty. If a reliable technology can be found to control particulates, and at a reasonable cost, prudence would suggest erring on the side of the error that can be reversed.

The makers of emission control technologies say they already have the answer, a device called the trap oxidizer, that lowers emissions to less than the proposed standard for 1985. The trap also--it is claimed--eliminates diesel odor, does not affect fuel economy, and adds about $100 to the cost of vehicles selling for $10,000. The auto industry doubts the trap's long-term reliability and fears that it won't be ready in sufficient numbers in time. The trap's proponents say their invention makes the United States the world leader in what will be a lucrative export market. Detroit raises foreign competition as a reason for postponing diesel regulation.

It all sounds very like the debates of a decade ago over the catalytic converter and the other changes needed to meet clean-air standards for gasoline engines. The success of that technology argues in favor of diesel controls until the health questions have been resolved.