Not the overwhelming majority, but nearly 3 to 1 are Democrat (71 to 29) judging solely by campaign donations.
http://verdantlabs.com/politics_of_professions/
It depends mostly on location. I'm in Tennessee and among engineers within this area, I'm a raging far left liberal. You literally could not find a handful of Democrat engineers in Chattanooga outside of government employ and software engineering. Even within AEC and manufacturing nationally, I'm on the liberal side judging my interaction with others, and I'd be considered a raging far right loon in LA or Austin. Even engineers we've hired from up North tend to be quite conservative, even though that doesn't ever come up in interviews. When one includes software engineers and government employees is where the total really begins to tilt left.
Further food for thought: Machine Design and Electronics Design found that 42.1% of engineers are Republican versus 33.7% independent and a mere 14.5% Democrat.
http://machinedesign.com/news/politics-engineers
It's also worth pointing out that there may be reason to obscure political affiliation, especially for publicly available data such as political contributions. Speaking from experience, a liberal engineer here is considered odd. In places like Silicone Valley, anyone with such beliefs might face far more than being labeled odd; being known as a Trump supporter or Republican could easily derail a career.