Good mechanical design combines form, fit, and function. It also includes analysis, material choice, manufacturing efficiency, and technology. It’s a complex process that starts with a need.
Necessity and invention mark the start and end of design. Necessity defines the problem, while invention is the final design result.
Designers must balance costs, manufacturability, durability, and the product’s purpose. Often, key design features conflict with one another.
Take automobile design as an example. In the mid-1980s, I owned a Honda Civic. This two-door hatchback had a 1.5 L carburetor-equipped engine without air conditioning. With little effort, I could get over 40 miles per gallon. At the same time, Volkswagen made a diesel car that reached 50 miles per gallon or more.
Now, my 2000 Honda Civic has a 1.6 L engine that is larger in size, but it weighs about 2,500 pounds. It only manages 40 miles per gallon on long freeway drives. The old Civic weighed around 1,700 pounds and had about 72 hp. The newer model, with over 110 hp, offers more safety and comfort. To me, the trade-off in efficiency is worth the added comfort and safety of a bigger car.
Could we get the same safety and comfort from lighter materials and better designs? Yes, but this would raise costs. Composites are the best materials for weight and strength, plus they resist road salt. Today’s engine and transmission technology can help us create a car that gets over 50 miles per gallon. But it will likely incur a higher cost. This could cut market share.
Another option is aluminum. It’s one-third the weight of steel and can match or exceed many steel strengths used in cars. It’s resistant to corrosion and easier to recycle than composites. Recycling composites can be a costly challenge.
Lighter cars have many benefits. A smaller engine could use less weight. Smaller engines are lighter and use less fuel. This means they need smaller gas tanks and lighter parts. This cycle can further reduce weight and costs, boosting profitability and market share.
A diesel engine could also work with current designs to offer the same or better efficiency. For example, the Euro Civic has a 1.6 L diesel engine that gets over 60 MPG. Unfortunately, these models aren’t sold in the USA. This efficiency is why I’d consider buying a new car. If that car were lighter, could it reach 70 MPG?
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org