Tycoon game simulating car industry (you run a car manufacturing company).

Before we start, to avoid confusion let's define two terms COMPONENETS and TECHNOLOGIES. When designing new cars you design COMPONENTS (like engine, chassis) which are later used by the cars you make. There are also TECHNOLOGIES which you invent (like +10% to engine power). So, TECHNOLOGIES affect COMPONENTS.

In this topic I'm not interested in components, it's a separate thing, but in RESEARCH of TECHNOLOGIES (using some sort of research points  or engineers or other mechanic). How to make an interesting RESEARCH TECHNOLOGIES mechanic for a car tycoon game?

Typically it's done like that in car tycoon games: you hire X engineers, then over time new technologies are unlocked and you use -/+ to assign engineers to research those technologies. Once done you have to reassign engineers (which is tedious and I don't like it).

Another approach (more 4X like) is to have a classic tech tree and research points which you spend to unlock techs.

You could divide technologies into X categories, then hire engineers that only produce research-points for one category.

(or better build laboratoria who cost upkeep/wages but also investment so the player isn't put in the situation that it would be profitable to micromanage firing/rehiring engineers)

Building on Dramolion's comment, your X technology categories (engine tech, materials, glass tech, metals, design, market, etc...) could be researched separately by a set of different buildings (labs or R&D buildings). Each could have a tech tree, and could require a different number of differently skilled people (so not only engineers but scientists, chemists, mechanics, artists, etc...). These buildings could themselves manage employees - and to speed up the process of a tech research, player could assign more people to work at each of them.