example.cs

class CarEngineData
{
    public string Displacement { get; }
    public string Horses { get; }

    public EmployeeTaxData(string displacement, string horses)
    {
        Displacement = displacement;
        Horses = horses;
    }

    // ...
}

class Car
{
    public string Model { get; }
    public string Brand { get; }
    public CarEngineData EngineData { get; }

    public Car(string model, string brand)
    {
        Model = model;
        Brand = brand;
    }

    public void SetEngine(string displacement, double horses)
    {
        EngineData = new CarEngineData(displacement, horses);
    }

    // ...
}